docs: update runtime architecture ownership (#697)

#### Overview

Update the Architecture page to reflect current runtime ownership and delivery behavior.

- [x] I confirm this contribution is my own work, or I have the right to submit it under this project's license.
- [x] I searched existing issues and open pull requests, and this does not duplicate existing work.

#### Details

- Show the CLI and language bindings as hosts of the shared Relay runtime.
- Show plugin components as configuration-installed behavior instead of presenting Adaptive as a peer layer.
- Separate the active scope stack from parent-linked event records.
- Distinguish awaited async middleware from background subscriber delivery.
- Add concrete middleware and exporter examples at first mention and remove the generic related-links section.

#### Where should the reviewer start?

Start with the architecture diagram and the middleware and subscriber sections in `docs/about-nemo-relay/architecture.mdx`.

#### Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

- Closes [RELAY-621](https://linear.app/nvidia/issue/RELAY-621/update-the-architecture-page-around-current-runtime-ownership)



## Summary by CodeRabbit

* **Documentation**
  * Expanded architecture guidance for CLI and runtime integration.
  * Clarified plugin components, middleware behavior, observability, guardrails, event handling, scopes, and managed execution.
  * Added diagrams for scope stacks, middleware resolution, and managed calls.
  * Documented parent-linked events and asynchronous subscriber delivery.
  * Updated terminology and related links for improved clarity.
  * Removed outdated sections and streamlined the overall architecture reference.

Authors:
  - Alex Fournier (https://github.com/afourniernv)
  - Will Killian (https://github.com/willkill07)

Approvers:
  - https://github.com/lvojtku
  - Will Killian (https://github.com/willkill07)

URL: #697

Author: afourniernv

docs/about-nemo-relay/architecture.mdx

@@ -8,18 +8,24 @@ import { MermaidStyles } from "@/components/MermaidStyles";
 {/* SPDX-FileCopyrightText: Copyright (c) 2026, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
 SPDX-License-Identifier: Apache-2.0 */}
 
-This page explains how NeMo Relay connects scopes, middleware, plugins, events,
-subscribers, and exporters.
+This page explains how NeMo Relay connects [scopes](/about-nemo-relay/concepts/scopes),
+[middleware](/about-nemo-relay/concepts/middleware),
+[plugins](/about-nemo-relay/concepts/plugins),
+[events](/about-nemo-relay/concepts/events), and
+[subscribers](/about-nemo-relay/concepts/subscribers).
 
 ## Architecture Diagram
 
-This diagram connects the runtime pieces to the layers they inhabit.
+This diagram shows how runtime hosts and integrations reach the shared Rust
+runtime. Plugin components install reusable behavior, and subscribers deliver
+event snapshots to in-process consumers or external backends.
 
 <MermaidStyles />
 
 ```mermaid
 flowchart TB
-    subgraph AppLayer[Framework Integrations and Application Code]
+    subgraph AppLayer[Runtime Hosts and Integrations]
+        CLI[NeMo Relay CLI]
         App[Application Code]
         Framework[Framework Integration]
     end
@@ -28,9 +34,10 @@ flowchart TB
         Bindings[Language Bindings]
     end
 
-    subgraph PluginLayer[Plugin and Adaptive Layer]
+    subgraph PluginLayer[Plugin System and Components]
         PluginSystem[Plugin System]
-        Adaptive[Adaptive Component]
+        Components[Adaptive / Observability / Guardrail Components]
+        PluginSystem -->|activates| Components
     end
 
     subgraph CoreLayer[Core Runtime]
@@ -54,10 +61,11 @@ flowchart TB
     App -. direct use .-> Bindings
     App -->|registers and configures| PluginSystem
     Framework -->|calls| Bindings
+    CLI -->|hosts| Core
+    CLI -->|loads configuration| PluginSystem
     Bindings --> Core
-    Adaptive -->|activates via| PluginSystem
-    PluginSystem -->|installs| Registry
-    PluginSystem -->|installs| Subs
+    Components -->|register middleware| Registry
+    Components -->|register subscribers| Subs
     Core -->|updates| Scope
     Core -->|resolves| Registry
     Core -->|emits| Events
@@ -71,41 +79,55 @@ flowchart TB
     class CoreLayer grey-hint;
     class ObsLayer grey-hint;
     class RuntimeState grey-lightest;
+    class CLI purple-lightest;
     class App purple-lightest;
     class Framework yellow-lightest;
     class Bindings green-lightest;
     class PluginSystem green-light;
-    class Adaptive blue-lightest;
+    class Components blue-lightest;
     class Core green-light;
     class Scope green-light;
     class Registry green-light;
     class Events green-light;
+    class Dispatcher green-light;
     class Subs green-light;
     class Backends grey-light;
 ```
 
-Adaptive appears here as a built-in plugin component rather than a separate runtime model because it activates through the same plugin lifecycle.
+Adaptive behavior is one plugin component installed through the same lifecycle
+as observability or PII redaction. It is not a separate runtime layer.
 
 ## Runtime Model
 
-NeMo Relay combines a small number of runtime pieces into one shared execution model:
-
-- The **scope stack** answers where work belongs
-- The **middleware registries** answer what should happen around that work
-- The **plugin system** installs reusable runtime behavior from configuration
-- The **event stream** records what happened
-- The **async subscriber dispatcher** delivers event snapshots after emission
-- **subscribers** consume those events
-
-Every emitted scope, tool, LLM, or mark event attaches to the active scope stack. Every managed tool or LLM call resolves the currently visible middleware before it executes.
+NeMo Relay combines a small number of runtime pieces into one shared execution
+model:
+
+- The [**scope stack**](/about-nemo-relay/concepts/scopes) answers where work
+  currently belongs.
+- The [**middleware registries**](/about-nemo-relay/concepts/middleware) answer
+  what should happen around that work.
+- The [**plugin system**](/about-nemo-relay/concepts/plugins) installs reusable
+  runtime behavior from configuration.
+- The [**event records**](/about-nemo-relay/concepts/events) preserve what
+  happened and how work was related.
+- The [**async subscriber dispatcher**](/about-nemo-relay/concepts/subscribers#waiting-for-delivery)
+  delivers event snapshots after emission.
+- [**Subscribers and exporters**](/about-nemo-relay/concepts/subscribers#common-subscriber-roles)
+  consume those snapshots.
+
+Every managed tool or LLM call resolves the middleware visible from the active
+scope before it executes. When the runtime emits an event, it records the active
+scope UUID as parentage. The scope stack changes as work opens and closes; the
+parent-linked event records remain available to subscribers.
 
 ## Main Runtime Pieces
 
 These components are the primary building blocks that make up the runtime model.
 
 ### Scope Stack
 
-The active scope stack defines the ownership tree for runtime work. It establishes:
+The active scope stack defines the current context for runtime work.
+It establishes:
 
 - Parent-child relationships between events
 - Scope-local visibility for middleware and subscribers
@@ -114,21 +136,54 @@ The active scope stack defines the ownership tree for runtime work. It establish
 
 ### Middleware Registries
 
-The middleware registries hold the active intercepts and guardrails for tool and LLM execution. Managed helpers read those registries before invoking the real callback.
+The middleware registries hold the active intercepts and guardrails for tool and
+LLM execution. Request intercepts can rewrite real requests, conditional
+guardrails can reject execution, and sanitize guardrails can change emitted
+observability payloads. Managed helpers read those registries before invoking
+the real callback.
+
+Async middleware callbacks are awaited as part of managed execution. The
+managed call does not advance past that middleware callback until the callback
+returns.
 
 ### Plugin System
 
-The plugin system installs reusable runtime components from configuration. A plugin can register middleware, subscribers, or related behavior without requiring each application call site to do the work manually.
+The plugin system installs reusable runtime components from configuration. A
+plugin can register middleware, subscribers, or related behavior without
+requiring each application call site to repeat the setup.
 
 ### Event Emission
 
-The runtime emits structured events for scopes, tools, LLMs, and named marks. Those events are the canonical record of runtime behavior. Native Rust, Python, Node.js, and FFI event-producing APIs enqueue subscriber work and return without waiting for subscriber callbacks or exporter work.
+The runtime emits structured events for scopes, tools, LLMs, and named marks.
+Those parent-linked records are the canonical history of runtime behavior.
+Native Rust, Python, Node.js, and FFI event-producing APIs enqueue subscriber
+work and return without waiting for subscriber callbacks or exporter work.
 
 ### Subscribers and Exporters
 
-Subscribers consume the event stream through the background dispatcher. Some subscribers stay in-process. Others export that stream into files or tracing systems. Use the binding flush API when a test or shutdown path must wait for already-queued subscriber work.
-
-## Two Axes of Runtime State
+Subscribers consume event snapshots through the background dispatcher. Some
+stay in process. Exporter subscribers can write
+[ATOF JSONL](/configure-plugins/observability/atof), project events into
+[ATIF trajectories](/configure-plugins/observability/atif), or emit
+[OpenTelemetry traces](/configure-plugins/observability/opentelemetry).
+Typical destinations include in-process application logic, local files and
+artifact pipelines, OTLP-compatible observability backends, and evaluation or
+visualization tools.
+
+This delivery happens after event submission and is separate from awaited async
+middleware. Use the binding flush API when a test or shutdown path must wait for
+already queued subscriber work. For manually registered exporters, follow the
+exporter's documented teardown order before process exit. The
+[ATOF](/configure-plugins/observability/atof),
+[OpenTelemetry](/configure-plugins/observability/opentelemetry), and
+[OpenInference](/configure-plugins/observability/openinference) exporters
+deregister to stop new deliveries, force-flush queued work, and then shut down.
+The [ATIF](/configure-plugins/observability/atif) export operation drains queued
+subscriber work before taking its snapshot; after export, deregister the
+subscriber and clear the exporter state. Clearing plugin configuration owns
+teardown for plugin-installed exporters.
+
+## Where Runtime State Lives
 
 Runtime state is easiest to understand by separating ownership from process-wide
 registration.
@@ -142,6 +197,29 @@ The scope stack defines:
 - When scope-local registrations are cleaned up
 - Whether concurrent requests stay isolated
 
+Only scopes are pushed onto the stack. Managed LLM and tool calls emit lifecycle
+records that use the current top scope as `parent_uuid`; they do not become
+stack entries. This example shows an agent scope with a nested function scope
+named `turn-a`.
+
+```mermaid
+flowchart BT
+    Root["BOTTOM: Implicit root<br/>uuid = root-a"]
+    Agent["Agent scope<br/>uuid = agent-a<br/>parent_uuid = root-a"]
+    Turn["TOP: Function scope (turn)<br/>uuid = turn-a<br/>parent_uuid = agent-a"]
+
+    Root -->|"push agent-a"| Agent
+    Agent -->|"push turn-a"| Turn
+
+    class Root grey-light;
+    class Agent,Turn green-light;
+```
+
+With `turn-a` at the top, managed LLM and tool records receive
+`parent_uuid = turn-a`, so they are siblings in the event tree. Popping
+`turn-a` returns `agent-a` to the top and removes registrations owned by the
+turn. A concurrent request uses a separate stack.
+
 ### Middleware Ownership
 
 Middleware exists at two levels:
@@ -151,6 +229,43 @@ Middleware exists at two levels:
 
 That split lets long-lived defaults coexist with request-specific or task-specific behavior.
 
+The following diagram shows how global and scope-local middleware are resolved
+for two concurrent requests.
+
+```mermaid
+flowchart TB
+    Global["Global middleware<br/>process-wide"]
+
+    subgraph RequestA["Request A scope stack"]
+        LocalA["Scope-local middleware<br/>owned by agent-a"]
+        ResolveA["Merge visible entries<br/>and order by priority"]
+        CallA["Managed call A"]
+        CloseA["Close agent-a"]
+        LocalA --> ResolveA --> CallA
+        CloseA -.->|"removes"| LocalA
+    end
+
+    subgraph RequestB["Request B scope stack"]
+        LocalB["Scope-local middleware<br/>owned by agent-b"]
+        ResolveB["Merge visible entries<br/>and order by priority"]
+        CallB["Managed call B"]
+        LocalB --> ResolveB --> CallB
+    end
+
+    Global --> ResolveA
+    Global --> ResolveB
+
+    class Global blue-lightest;
+    class RequestA,RequestB grey-lightest;
+    class LocalA,LocalB green-lightest;
+    class ResolveA,ResolveB green-light;
+    class CallA,CallB yellow-lightest;
+    class CloseA grey-light;
+```
+
+Global entries are visible to both calls. Each scope-local entry is visible
+only through its owning stack and is removed when that scope closes.
+
 ## Managed Execution Pipeline
 
 Managed tool and LLM execution follows the same high-level order:
@@ -162,36 +277,53 @@ Managed tool and LLM execution follows the same high-level order:
 5. The user callback runs.
 6. Sanitize-response guardrails can rewrite the emitted end-event payload.
 
+The following sequence shows where each middleware family runs during a managed
+call.
+
+```mermaid
+sequenceDiagram
+    autonumber
+    actor Caller as Application / Framework
+    participant Runtime as NeMo Relay Runtime
+    participant Conditional as Conditional Guardrails
+    participant Request as Request Intercepts
+    participant Sanitizers as Request / Response Sanitizers
+    participant Execution as Execution Intercepts
+    participant Callback as Real Callback
+    participant Dispatcher as Async Subscriber Dispatcher
+
+    Caller->>Runtime: managed tool or LLM call
+    Runtime->>Conditional: evaluate real request
+    alt rejected
+        Conditional-->>Runtime: rejection
+        Runtime-->>Caller: guardrail error
+    else allowed
+        Conditional-->>Runtime: continue
+        Runtime->>Request: transform real request
+        Request-->>Runtime: intercepted request
+        Runtime->>Sanitizers: sanitize event-only request copy
+        Sanitizers-->>Runtime: start-event payload
+        Runtime->>Dispatcher: enqueue start event
+        Runtime->>Execution: invoke intercept chain
+        alt intercept replaces execution
+            Execution-->>Runtime: replacement result
+        else intercept calls next
+            Execution->>Callback: invoke real callback
+            Callback-->>Execution: real result
+            Execution-->>Runtime: real result
+        end
+        Runtime->>Sanitizers: sanitize event-only response copy
+        Sanitizers-->>Runtime: end-event payload
+        Runtime->>Dispatcher: enqueue end event
+        Runtime-->>Caller: return real result
+    end
+```
+
 Two distinctions matter:
 
 - Intercepts affect the real execution path
 - Sanitize guardrails affect the emitted observability payload
 
-For the expanded request-to-response runtime path, including streaming and subscriber handoff, refer to [Middleware](/about-nemo-relay/concepts/middleware#detailed-execution-flow).
-
-## Runtime Layers
-
-From bottom to top, NeMo Relay is organized as:
-
-1. The Rust core runtime
-2. The plugin and adaptive layer
-3. Language bindings
-4. Framework integrations and application code
-5. Subscribers and observability backends
-
-The details of a binding can vary, but the conceptual model stays the same across those layers.
-
-## Design Goal
-
-NeMo Relay is designed so that application developers, framework integrators, plugin authors, and observability consumers all reason about the same runtime semantics. One conceptual model should remain stable even when the binding or integration style changes.
-
-## Related Concepts
-
-The following concepts are related to this architecture:
-
-- [Scopes](/about-nemo-relay/concepts/scopes)
-- [Middleware](/about-nemo-relay/concepts/middleware)
-- [Events](/about-nemo-relay/concepts/events)
-- [Subscribers](/about-nemo-relay/concepts/subscribers)
-- [Plugins](/about-nemo-relay/concepts/plugins)
-- [Codecs](/about-nemo-relay/concepts/codecs)
+For the expanded request-to-response runtime path, including streaming and
+subscriber handoff, refer to
+[Managed Execution Order](/about-nemo-relay/concepts/middleware#managed-execution-order).