test(build): add coverage thresholds and coverage notes

Add global coverage thresholds to vite.config.ts (80% stmts/lines/funcs,
75% branches) with an explicit exclude list mirroring test.exclude.
Add docs/coverage-notes.md documenting known-low files (useBuzzer.ts ~2%,
db/index.ts ~46%), the rationale for global-only thresholds, and c8 ignore
annotation conventions for untestable callsites.

Author: morbeo

docs/coverage-notes.md

@@ -0,0 +1,42 @@
+# Coverage Notes
+
+Files with intentionally low coverage, tracked here until their epics are complete.
+CI enforces **global** thresholds only — no per-file gates.
+
+## Known-low files
+
+| File | Stmts | Reason | Tracking |
+|---|---|---|---|
+| `hooks/useBuzzer.ts` | ~2% | Buzzer epic not yet implemented — WebRTC adjudication logic lives here but has no unit-testable surface until the GM control system is wired up | Buzzer epic |
+| `db/index.ts` | ~46% | Low-level Dexie helpers; untested paths are legacy migration scaffolding and emergency purge routines that require real IndexedDB environments | — |
+
+## Annotation conventions
+
+Use `/* c8 ignore next */` for a single uncovered line:
+```ts
+/* c8 ignore next */
+if (process.env.NODE_ENV === 'test') return
+```
+
+Use `/* c8 ignore start */` / `/* c8 ignore stop */` for a block:
+```ts
+/* c8 ignore start */
+// WebRTC teardown — untestable without a real peer connection
+peer.destroy()
+/* c8 ignore stop */
+```
+
+These annotations suppress v8 coverage for those lines so they do not
+drag down thresholds. Add a comment explaining *why* the block is excluded.
+
+## Thresholds (vite.config.ts)
+
+| Metric | Threshold |
+|---|---|
+| Statements | 80% |
+| Lines | 80% |
+| Functions | 80% |
+| Branches | 75% |
+
+Branches are set slightly lower because JSX ternaries and optional chaining
+generate branch entries that are rarely all exercised in unit tests.

vite.config.ts

@@ -56,6 +56,27 @@ export default defineConfig({
     coverage: {
       provider: 'v8',
       reporter: ['text', 'lcov'],
+      // Files excluded from coverage entirely (untestable boilerplate,
+      // entry points, or files covered by e2e rather than unit tests).
+      exclude: [
+        'src/main.tsx',
+        'src/pages/**',
+        'src/components/AdminLayout.tsx',
+        '**/*.d.ts',
+        'vite.config.ts',
+        'commitlint.config.js',
+      ],
+      // Global thresholds — CI fails if the whole suite drops below these.
+      // Per-file thresholds are intentionally omitted: files with known-low
+      // coverage (useBuzzer.ts, db/index.ts) are annotated with
+      // /* c8 ignore */ at the untestable callsites and tracked in
+      // docs/coverage-notes.md until their epics are complete.
+      thresholds: {
+        lines: 80,
+        functions: 80,
+        branches: 75,
+        statements: 80,
+      },
     },
   },
 })