build(ts): stripInternal so @internal test-only exports stay out of the shipped .d.ts (#2201 review R5)

computeReachingDefsDense/computeReachingDefsSparse are exported only for the
equivalence fuzz and tagged @internal, but `declaration: true` emitted them into
the public dist/**/*.d.ts. stripInternal removes any @internal-tagged export from
the declaration output.

This is repo-wide, which is the intended behavior: the same applies to every
other test-only @internal export (hf-env's withDownloadTimeout etc., worker-pool's
buildDispatchMessage/crashSignature, parse-impl's handleWorkerStartupFailure, the
logger/safe-parse test resets, and the new reaching-defs-graph SSA helpers) — all
of which are documented as not-public.

Verified:
- declaration emit succeeds with no TS4094/TS9006 ("cannot be named") errors;
- the @internal functions are gone from the emitted .d.ts (reaching-defs-graph.d.ts
  is now `export {};`), while public symbols (computeReachingDefs) remain;
- gitnexus-web — the only cross-package consumer — typechecks clean and imports
  only from gitnexus-shared, never from gitnexus internals;
- runtime .js and the vitest/tsx tests are source-based, so unaffected.

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

gitnexus/tsconfig.json

@@ -12,6 +12,7 @@
     "resolveJsonModule": true,
     "forceConsistentCasingInFileNames": true,
     "declaration": true,
+    "stripInternal": true,
     "types": ["node"]
   },
   "include": ["src/**/*"]