✨ 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.

package.json

@@ -49,6 +49,8 @@
         "test": "npm run check:tsc && node --test --strip-types \"src/**/*.test.ts\"",
         "test:ci": "c8 --reporter=text --reporter=lcov --reporter=html node --test --strip-types \"src/**/*.test.ts\"",
         "test:unit": "node --test --strip-types \"src/**/*.test.ts\"",
+        "watch": "npm run watch:test",
+        "watch:test": "node --test --watch --strip-types \"src/**/*.test.ts\"",
         "publish:attw": "attw . --pack --profile esm-only",
         "publish:publint": "publint",
         "use:git-commit-message": "cp commit-message-template .git/COMMIT_EDITMSG || true"