✨ Add watch tier with watch:test child

Adds the earliest feedback tier to the ladder: `npm run watch`
re-runs tests on file save, started manually in a dedicated
terminal pane. Built on Node 26's native `node --test --watch`
(no new dependency).

Structure follows the existing prefix convention:
- `watch:test` — runs the test suite in watch mode
- `watch` — umbrella that aggregates `watch:*` children
  (currently just `watch:test`; future `watch:oxlint` etc.
  would aggregate here, switching to concurrent execution)

CONTRIBUTING.md documents the new tier in three places: the
prefix convention list, the feedback tiers table (new top row),
and a 'Why these splits?' bullet explaining why watch is a
manual tier rather than a hook. README's Development section
gains a one-line pointer.

README.md

@@ -6,6 +6,7 @@ Pattern matching for TypeScript/ESM environments (F#-style, not regex).
 
 - **Build:** `npm run build`
 - **Test:** `npm run test`, `npm run test:ci`
+- **Watch:** `npm run watch` (re-runs tests on file save; the earliest feedback tier)
 - **Checks:** `npm run check`, `npm run fix`
 - **Individual fixes:** `npm run fix:oxfmt`, `npm run fix:oxlint`