[inth3shadows]

What

This adds a CI job, TS grammar differential, that exists only in the fork. It treats tsc as the ground truth for TypeScript syntax and checks the vendored tree-sitter-typescript and tsx wasm grammars against it.

• Oracle. tsc's verdict comes from ts.createSourceFile(...).parseDiagnostics, syntax only. Tree-sitter's verdict is rootNode.hasError. The job runs under --liftoff-only, the same flag as production (a self re-exec, checked against src/extraction/wasm-runtime-flags.ts). Canaries check that the tsc oracle (both TS and TSX) and both grammars can still tell good input from bad. If either can't, the job fails before it does any corpus work.
• Corpora. Effect, Zod, Excalidraw, VS Code src/, and the in-repo __tests__/fixtures/ts-syntax/. The external ones are pinned by SHA and cached under $RUNNER_TEMP.
• Gate (scripts/ts-differential.baseline.json records each divergent file's first error row):
  • exit 1 if the run finds any of these:
    • a file that tsc accepts, tree-sitter rejects, and the baseline doesn't list
    • in a pinned corpus, a baselined file whose first error moved to an earlier line
    • a fixture that tsc rejects
    • a non-gap fixture that diverges
  • exit 2 if the job can't trust its inputs:
    • the tsc or web-tree-sitter version doesn't match the baseline
    • a canary fails
    • a corpus is empty
    • a baselined path, or a whole baselined corpus, has vanished
    • a baseline row is malformed
    • any other setup error
  • Reported but not failing: baselined files that now agree with tsc, and errors that moved later. Both suggest a re-baseline with --update.
• Known gaps. Each of these fixtures holds one construct: variance.ts (tree-sitter-typescript#370), static accessor, import attributes on export {..} from and on export * from, using in a for-of head, and await using in a for-await head. A grammar that fixes one reports it as now-agree, which is how a candidate fork is compared with ours.

Scope: wasm only. The native kernel's grammar is not exercised here. Wasm/kernel parity is scripts/kernel-parity.mjs's job.

Background: tree-sitter-typescript has had no commit since 2025-01-30 (tree-sitter/tree-sitter#6091), and we carry grammar patches (colbymchenry#3226).

Verified

Full local run against current main's wasm (tsc 5.9.3, web-tree-sitter 0.25.10). It exits 0:

corpus      files  DIVERGENT  new  worse
effect       2056        239    0      0
zod           515          4    0      0
excalidraw    695          5    0      0
vscode      11267         11    0      0
local          25          6    0      0

Two review rounds (/code-review plus a code-reviewer agent) found 21 issues in total. All are fixed except one accepted low-severity item: SIGTERM sent to the parent orphans the relaunched child. Each fix has a before/after check: a new fixture missing from the baseline exits 1; a version mismatch, a renamed fixture, a 0-file corpus, a blind oracle, flag drift and a malformed row each exit 2.