build/CI: no `prettier --check` gate; formatting drift accumulating on main

[bpowers]

Problem

Neither the pre-commit hook (scripts/pre-commit) nor CI runs a prettier formatting check for TypeScript/JavaScript. As a result, files can be committed prettier-dirty, and drift has already accumulated on main.

The pre-commit hook only checks Rust formatting (the [rust] Checking formatting... step). For TypeScript it runs eslint, but eslint uses eslint-config-prettier solely to disable rules that conflict with prettier (eslint.config.shared.js:68-69, comment: "Disable rules that conflict with prettier") -- it does not enforce prettier formatting. A grep of .github/workflows/ for prettier/format --check/needs-format returns nothing.

Evidence

Two independent observations during Phase 2 of the @simlin/engine wasm backend work confirmed this:

1. Newly-created Phase 2 files were prettier-dirty yet passed the pre-commit hook.

2. A whole-tree check run from src/engine currently flags 6 files that are not touched by the engine-wasm-sim branch and are byte-identical to main (verified again while filing):

   $ cd src/engine && npx prettier --check 'src/**/*.ts' 'tests/**/*.ts'
   [warn] src/internal/project.ts
   [warn] src/project.ts
   [warn] src/worker-backend.ts
   [warn] src/worker-protocol.ts
   [warn] src/worker-server.ts
   [warn] tests/direct-backend.test.ts
   [warn] Code style issues found in 6 files. Run Prettier with --write to fix.
   

These 6 are pre-existing drift on main, not introduced by the current work; they were intentionally left untouched to avoid mixing unrelated reformatting into the wasm-backend feature's review. There is likely similar drift in other packages (core, diagram, app, server, serve-web) -- the 6 above are just what surfaced from the engine-package check.

Why it matters

• Developer experience / review hygiene: prettier-dirty files merge silently, then unrelated PRs get noisy reformatting diffs (or reviewers flag formatting that the tooling should have caught).
• The repo already intends prettier to be the formatter (pnpm js-format / pnpm format exist and write changes), but nothing verifies it on commit or in CI -- so the intent isn't enforced.

Components affected

• scripts/pre-commit
• .github/workflows/ (CI)
• All TypeScript packages: engine, core, diagram, app, server, serve-web

Suggested resolution

Two parts, ordering matters -- clean up first, then add the gate, or the gate's first run will be noisy:

1. One-time prettier --write sweep to clear existing drift (the 6 engine files above, plus a whole-repo pnpm js-format pass). Do this as its own commit so the formatting-only churn is isolated.

2. Add a prettier --check gate to the pre-commit hook and CI. A pnpm js-needs-format script already exists in the root package.json (find src -name '*.ts' -o -name '*.tsx' | egrep -v '/(lib(\.(browser|module))?|core)/' | xargs prettier -l) and is currently wired into nothing -- it's the natural gate to reuse, mirroring how pnpm rust-needs-format (cargo fmt -- --check) is already enforced by the Rust formatting step.

   Caveat to resolve when wiring it: the existing js-needs-format script excludes core/ directories via its egrep -v filter, so a faithful reuse would skip those. Decide whether to broaden the glob (drop the core exclusion, which appears aimed at generated lib*/ output rather than src/core) or accept the gap, and document the choice.

Discovered

Identified during Phase 2 of the @simlin/engine wasm simulation backend work (engine-wasm-sim branch). Tracking only -- not fixed as part of that work to keep the feature's diff scoped.

This is distinct from tech-debt.md item #60 ("No CI feature matrix"), which is about a Cargo feature matrix for Rust builds, not TypeScript prettier formatting.