vt: own ghostty libghostty-vt via ix-vt (#332)

Draft. Owns the real terminal-emulation engine: ghostty's libghostty-vt,
wrapped in our own public `ix-vt` crate. New files only; does not touch
the vt100 path (the companion dashboard PR owns that). The vt100 to
ix-vt swap is a later third PR.

## What this does

- **`packages/vt/libghostty-vt`** (+ `lib/libghostty-vt.nix`): Nix
derivation building ghostty's `libghostty-vt` static lib +
self-contained dylib via `zig build -Demit-lib-vt=true` against a pinned
ghostty source, with zig deps vendored for a network-free build. zig
0.15.x only.
- **`packages/vt/ix-vt-sys`**: raw FFI, bindgen-generated `bindings.rs`
(495 symbols) over the ghostty C headers; `build.rs` links the Nix-built
lib.
- **`packages/vt/ix-vt`**: safe wrapper. `Terminal::new` / `vt_write` /
`resize` / a render-state snapshot exposing styled cells (fg/bg palette
+ RGB, bold/italic/underline/inverse), scrollback, and the cursor
(position, visible, blinking, visual style bar/block/underline/hollow).
Round-trip test in `tests/round_trip.rs`.

## Why own it instead of the community crate

The community `libghostty-vt` crate's build.rs runs native `zig build`
and fetches deps over the network, which fails the Nix purity and darwin
link gates. The capability is identical, so we own the build, not the
API. Proven by a spike: the lib built in ~57s and round-tripped a
DECSCUSR bar cursor, cursor position, and `#cc6666` styled cells through
the C API.

## Status

- Committed and pushed. Left: confirm `nix build .#libghostty-vt` + `nix
build .#ix-vt` + the round-trip test green under Nix, then `nix run
.#lint`, then ready for review.

## Notes for the reviewer

- zig 0.15.x exactly (requireZig is a comptime gate; 0.16 is rejected).
- The static `.a` does not bundle its C++ deps (needs sibling
`libhighway`/`libsimdutf`/`libutfcpp` + `-lc++`); the dylib is
self-contained, so `build.rs` prefers it.
- zig deps vendored via zon2nix.
- Minor: this branch also touched
`packages/mcp/src/server_instructions.txt` (+2 lines), which the
companion PR also edits. Resolve at merge (let the dashboard PR win).

Made with Claude (claude-opus-4-8).

---------

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

Author: andrewgazelka

Cargo.lock

@@ -22,6 +22,8 @@ members = [
   "packages/tui-dashboard",
   "packages/tui-node",
   "packages/tui-py",
+  "packages/vt/ix-vt",
+  "packages/vt/ix-vt-sys",
 ]
 resolver = "3"
 

Cargo.toml

@@ -50,6 +50,18 @@
       inputs.nixpkgs.follows = "nixpkgs";
       inputs.rust-overlay.follows = "rust-overlay";
     };
+
+    # Ghostty's terminal VT engine, consumed as a source tree (not a flake) so
+    # `packages/vt/libghostty-vt` owns the build. Pinned to the commit the
+    # local clone validated against; `requireZig` in `build.zig.zon` is exact
+    # minor (0.15.x), so the build uses `pkgs.zig_0_15`. The pinned tree ships
+    # `build.zig.zon.nix` (zon2nix output), which vendors every lazy Zig
+    # dependency with SRI hashes for a network-free build. Bump by repointing
+    # this rev and running `nix flake update ghostty`.
+    ghostty = {
+      url = "github:ghostty-org/ghostty/fd49716ea2084108aa098db390931c007495a1ab";
+      flake = false;
+    };
   };
 
   outputs =
@@ -60,6 +72,7 @@
       hermes-agent,
       symphony,
       clippy-fork,
+      ghostty,
       ...
     }:
     let
@@ -104,6 +117,7 @@
           hermes-agent
           symphony
           clippy-fork
+          ghostty
           ;
       };
       devSystems = [

flake.lock

@@ -459,17 +459,84 @@ let
       meta ? { },
       passthru ? { },
     }:
+    selectRootWithTests workspace {
+      rootDrv = workspace.binaries.${binary} or workspace.default;
+      inherit
+        packageName
+        testTargets
+        doctestTargets
+        includeTestCases
+        meta
+        passthru
+        ;
+      defaultTestTargets = [ binary ];
+    };
+
+  /**
+    Pick a library target from a pre-built `buildWorkspace` plus its test and
+    doctest derivations, ready for `passthru.tests` consumption.
+
+    The library version of `selectBinaryWithTests`, for crates that ship a
+    `lib` target rather than a binary. `library` is the crate's library unit
+    key (Cargo's underscored name, e.g. `ix_vt`); `packageName` is the Cargo
+    package name used to look up test targets (e.g. `ix-vt`).
+  */
+  selectLibraryWithTests =
+    workspace:
+    {
+      library,
+      packageName,
+      testTargets ? null,
+      doctestTargets ? null,
+      includeTestCases ? true,
+      meta ? { },
+      passthru ? { },
+    }:
+    selectRootWithTests workspace {
+      rootDrv =
+        workspace.libraries.${library}
+          or (throw "selectLibraryWithTests: no library `${library}` in workspace; available: ${
+            lib.concatStringsSep ", " (builtins.attrNames (workspace.libraries or { }))
+          }");
+      inherit
+        packageName
+        testTargets
+        doctestTargets
+        includeTestCases
+        meta
+        passthru
+        ;
+      defaultTestTargets = [ packageName ];
+    };
+
+  # Shared core for `selectBinaryWithTests` / `selectLibraryWithTests`: take a
+  # selected root derivation and assemble its `passthru.tests` from the shared
+  # workspace's test/doctest targets and policy checks.
+  selectRootWithTests =
+    workspace:
+    {
+      rootDrv,
+      packageName,
+      defaultTestTargets,
+      testTargets ? null,
+      doctestTargets ? null,
+      includeTestCases ? true,
+      meta ? { },
+      passthru ? { },
+    }:
     let
-      binaryDrv = workspace.binaries.${binary} or workspace.default;
-      uncheckedBinary = binaryDrv.passthru.unchecked or binaryDrv;
+      uncheckedRoot = rootDrv.passthru.unchecked or rootDrv;
       namesForPackage =
         attrName: fallback:
         if builtins.hasAttr attrName workspace && builtins.hasAttr packageName workspace.${attrName} then
           workspace.${attrName}.${packageName}
         else
           fallback;
       selectedTestTargets =
-        if testTargets == null then namesForPackage "testTargetNamesByPackage" [ binary ] else testTargets;
+        if testTargets == null then
+          namesForPackage "testTargetNamesByPackage" defaultTestTargets
+        else
+          testTargets;
       selectedDoctestTargets =
         if doctestTargets == null then
           namesForPackage "doctestTargetNamesByPackage" [ ]
@@ -494,20 +561,20 @@ let
         flattenCaseTargets "" selectedTestTargets (workspace.tests or { })
         // flattenCaseTargets "doctest-" selectedDoctestTargets (workspace.doctests or { });
       tests = {
-        package = uncheckedBinary;
+        package = uncheckedRoot;
       }
       // flattenAllTargets "" selectedTestTargets (workspace.tests or { })
       // flattenAllTargets "doctest-" selectedDoctestTargets (workspace.doctests or { })
       // lib.optionalAttrs includeTestCases testCases;
     in
-    binaryDrv
+    rootDrv
     // {
-      meta = (binaryDrv.meta or { }) // meta;
+      meta = (rootDrv.meta or { }) // meta;
       passthru =
-        (binaryDrv.passthru or { })
+        (rootDrv.passthru or { })
         // passthru
         // {
-          tests = (binaryDrv.passthru.tests or { }) // policyChecks // (passthru.tests or { }) // tests;
+          tests = (rootDrv.passthru.tests or { }) // policyChecks // (passthru.tests or { }) // tests;
           inherit policyChecks;
           inherit (workspace) policy;
         };
@@ -581,6 +648,7 @@ in
     buildBinaries
     buildWorkspace
     selectBinaryWithTests
+    selectLibraryWithTests
     auditCargoLock
     generateUnitGraph
     generateUnitsNix

flake.nix

@@ -8,6 +8,7 @@
   hermes-agent,
   symphony,
   clippy-fork,
+  ghostty,
   cliArtifacts ? { },
 }:
 let
@@ -79,6 +80,7 @@ let
   };
   buildNpmVitest = import ./build-npm-vitest.nix;
   buildZigPackage = import ./build-zig-package.nix { };
+  buildLibghosttyVt = import ./libghostty-vt.nix { inherit lib writeNushellApplication; };
   uvLockFor =
     pkgs:
     import ./uv-lock.nix {
@@ -237,6 +239,7 @@ let
       rustWorkspaceFor
       cliArtifacts
       clippy-fork
+      ghostty
       ;
   };
 
@@ -257,6 +260,8 @@ let
       paths
       packageRegistry
       cargoUnitFor
+      ghostty
+      writeNushellApplication
       ;
   };
   rustWorkspace = rustWorkspaceFor pkgs;
@@ -277,6 +282,7 @@ let
       buildSvelteSite
       buildUvApplication
       buildZigPackage
+      buildLibghosttyVt
       cargoUnit
       goUnit
       languages
@@ -361,6 +367,7 @@ let
       buildSvelteSite
       buildUvApplication
       buildZigPackage
+      buildLibghosttyVt
       bunLockFor
       cargoUnit
       cargoUnitFor