docs: fix Pages publish + add strict link check to PR CI

The "Publish docs" workflow (mkdocs build --strict, push-to-main only) went
red after #448 merged, so the live site froze at the pre-merge build and none
of the V1->Control-Reconciler-Protocol changes appeared. Root cause: the
orphaned, stale docs/guide/extensibility-preview.md has 10 broken ../specs/*
links that abort the strict build. That page documents the retired provisional
surface (UseV1Protocol flag, [Experimental], legacy MountXxx path) and is fully
superseded by the Control Reconciler Protocol + Extending Reactor Controls
pages, so delete it.

Also surface the two new pages where they were still missing:
- index.md landing page "9. Under the hood" list (hand-maintained; never got
  the two pages) - added after Reconciliation, matching their 33.5/33.7 order
- mkdocs nav: moved both from after Architecture Overview to after
  Reconciliation so the sidebar and the index list agree

Prevent recurrence:
- ci.yml: the existing docs job (now "Docs build") runs `mkdocs build --strict`
  after compiling, mirroring the publish workflow, so a broken link fails the
  PR instead of the post-merge publish (the only place --strict ran before).
  One docs build job rather than two.
- ai-author-skill.md: author checklist + navigation guidance now require
  `python -m mkdocs build --strict` before submitting.

Verified: `python -m mkdocs build --strict` passes locally (exit 0) after the
deletion; remaining items are INFO-level anchor hints (non-fatal, pre-existing).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: codemonkeychris

.github/workflows/ci.yml

@@ -286,8 +286,8 @@ jobs:
       - name: Tier-lint
         run: dotnet run --project src/Reactor.Cli -- docs check-tier
 
-  docs-compile:
-    name: Docs compile
+  docs-build:
+    name: Docs build
     needs: changes
     if: needs.changes.outputs.non-md == 'true'
     runs-on: windows-latest
@@ -301,3 +301,21 @@ jobs:
 
       - name: Compile docs
         run: dotnet run --project src/Reactor.Cli -- docs compile --no-screenshots --ci
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+          cache: pip
+          cache-dependency-path: docs/requirements.txt
+
+      - name: Install MkDocs
+        run: pip install -r docs/requirements.txt
+
+      # Strict link/anchor check against the docs just compiled above,
+      # mirroring the Pages publish workflow (.github/workflows/docs.yml).
+      # `--strict` turns a broken cross-link or missing nav file into a
+      # failure, so it fails the PR instead of the post-merge publish (the
+      # only place --strict ran before). Folded into this job so there is a
+      # single docs build.
+      - name: mkdocs build --strict
+        run: python -m mkdocs build --strict

docs/_pipeline/ai-author-skill.md

@@ -491,6 +491,11 @@ these rules:
   by order number so readers can follow the learning path linearly.
 - **Link format.** Use relative `.md` paths: `[Getting Started](getting-started.md)`.
   Do not use absolute paths or `screenshot://` syntax for page links.
+- **Validate with `--strict`.** Before submitting, run `python -m mkdocs build --strict`
+  locally. The GitHub Pages publish *and* the PR CI both build with `--strict`, so any
+  broken cross-link or missing heading anchor aborts the build. Always link to a target
+  that exists; never rely on the non-strict build tolerating a dangling link, and never
+  silence the check — fix the link or the anchor.
 
 **Topic order for sequential navigation:** see the full 64-page index in
 the "Topic Ideas" table at the bottom of this file. Authors should always
@@ -907,3 +912,4 @@ doc to link from a reference page into the conceptual guide.
 - [ ] Page has a `## Next Steps` section with links to related and sequential topics
 - [ ] Readme links to all topic pages; all pages are reachable via link traversal
 - [ ] Run `mur docs compile --validate-only` to check all references resolve
+- [ ] Run `python -m mkdocs build --strict` — the publish step and PR CI both use it; a broken cross-link or missing heading anchor aborts the build. Fix the link, don't silence the check.

docs/_pipeline/templates/index.md.dt

@@ -158,6 +158,8 @@ does, not just how to use it.
 - **[Reactor vs XAML](reactor-vs-xaml.md)** — (also in §1) the architectural essay
 - **[Hooks Internals](hooks-internals.md)** — Hook slot table, dispatcher, closure capture
 - **[Reconciliation](reconciliation.md)** — Element-record diff, identity, the three phases
+- **[Control Reconciler Protocol](control-reconciler-protocol.md)** — The per-control handler protocol: `IElementHandler`, descriptors, children strategies, pooling
+- **[Extending Reactor Controls](extending-reactor-controls.md)** — Cookbook: add a native control end-to-end with a descriptor
 - **[Element Pool](element-pool.md)** — Allocation reduction under scroll-heavy lists
 - **[Effects Scheduling](effects-scheduling.md)** — When effects run; dep semantics; cleanup ordering
 - **[Threading and Dispatch](threading-and-dispatch.md)** — UI-thread invariants, trampoline, batched renders

docs/guide/README.md

@@ -184,6 +184,8 @@ does, not just how to use it.
 - **[Reactor vs XAML](reactor-vs-xaml.md)** — (also in §1) the architectural essay
 - **[Hooks Internals](hooks-internals.md)** — Hook slot table, dispatcher, closure capture
 - **[Reconciliation](reconciliation.md)** — Element-record diff, identity, the three phases
+- **[Control Reconciler Protocol](control-reconciler-protocol.md)** — The per-control handler protocol: `IElementHandler`, descriptors, children strategies, pooling
+- **[Extending Reactor Controls](extending-reactor-controls.md)** — Cookbook: add a native control end-to-end with a descriptor
 - **[Element Pool](element-pool.md)** — Allocation reduction under scroll-heavy lists
 - **[Effects Scheduling](effects-scheduling.md)** — When effects run; dep semantics; cleanup ordering
 - **[Threading and Dispatch](threading-and-dispatch.md)** — UI-thread invariants, trampoline, batched renders