feat(harness): agent-drivable dev harness and CI that gates

A coding agent could develop this repo's Go packages and could not
develop the application: every path to running YellowJacket ended in a
blocking GTK window, so 265 bound methods, 46 events, 33 component
directories and 13 stores had exactly one form of verification
available — `tsc --noEmit`.

The unlock is that `wails dev`'s dev server on :34115 serves the real
frontend with the real generated bindings against the same Go backend a
desktop window attaches to, so a plain Chromium under Xvfb gets a fully
functional app. Four test tiers now exist, cheapest first:

- `make ui-test` — 313 Vitest tests in a real browser in ~2 s, no app,
  no backend, no display. Works because `frontend/wailsjs/` is a pure
  passthrough to `window.go`/`window.runtime`, so faking just those two
  globals runs the real bindings and the real store code.
- `make test` — services in-process, asserting on the payload the
  frontend would receive, via a new `events.Emit` wrapper.
- `make dev-headless` + `playwright-cli` — the real app, driven
  interactively, with an event bridge on `window.__yjEvents` and a
  dev-only control surface at `/__test/`.
- `make e2e` — 19 of those flows frozen as Playwright specs.

`events.Emit(ctx, …)` replaces all 35 direct `runtime.EventsEmit` call
sites: wails' `getEvents` `log.Fatalf`s on any context without its
runtime, so those paths could not run under test and a background
worker could take the app down. Four packages had each hand-rolled the
same guard; nine more guarded on `ctx != nil`, which does not help.
`TestNoDirectRuntimeEmits` fails the build on a new one.

Fixtures are generated, not committed (`make testdata`), and seeds are
built by *running the app* — never by hand-writing config and DB rows,
which would be a second description of a valid YJ_HOME.

`.gitea/workflows/ci.yml` is the first workflow here that tests
anything; the other three only package, so `gitea_ci` reported only
packaging jobs and misled anyone asking whether a push was healthy.
Both jobs were prototyped to green in a bare ubuntu:24.04 container
before the YAML was written, which immediately caught `make lint`
linting three configurations that nothing builds: all three passes
omitted `webkit2_41`, so wails resolved webkit2gtk-4.0 — which Arch
still ships and Ubuntu 24.04 dropped.

Operational instructions live in `.pi/skills/yellowjacket-dev/`,
measured discoveries in `.planning/NOTES.md`, and architecture in
`CLAUDE.md` — split by tense, not by topic, because a topical split
gives every new fact two plausible homes. `make skill-check` fails a
commit if the skill cites a make target that does not exist.

e2e/specs/harness.spec.ts

@@ -0,0 +1,89 @@
+import {
+  test,
+  expect,
+  callBinding,
+  resetEvents,
+  waitForEvent,
+} from '../support/fixtures.js';
+
+/**
+ * The harness testing itself.
+ *
+ * If these fail, every other spec's failure is uninterpretable — a
+ * missing event could mean a broken feature or a broken recorder, and
+ * telling those apart afterwards is expensive.
+ */
+test.describe('harness', () => {
+  test('the app is the real app, not a mock', async ({ app }) => {
+    // All 11 bound services land on window.go through the dev server.
+    const services = await app.evaluate(() => Object.keys(window.go));
+
+    expect(services).toEqual(
+      expect.arrayContaining(['queue', 'player', 'library', 'explore']),
+    );
+
+    const state = await callBinding<{ tracks: unknown[] }>(
+      app,
+      'queue.Queue.GetState',
+    );
+
+    expect(state).toHaveProperty('tracks');
+  });
+
+  test('backend events are recorded, in order, with payloads', async ({
+    app,
+  }) => {
+    await resetEvents(app);
+    await callBinding(app, 'player.Player.SetVolume', [37]);
+
+    const ev = await waitForEvent(app, 'VolumeChanged');
+
+    expect(ev.data).toEqual([37]);
+    expect(ev.dir).toBe('in');
+  });
+
+  test('exactly one recorder is installed', async ({ app }) => {
+    // Listeners registered by one evaluate survive into the next, so a
+    // recorder that re-registers counts every event twice.  This is the
+    // regression test for that.
+    await resetEvents(app);
+    await callBinding(app, 'player.Player.SetVolume', [41]);
+    await waitForEvent(app, 'VolumeChanged');
+
+    const count = await app.evaluate(() =>
+      window.__yjEvents.count('VolumeChanged'),
+    );
+
+    expect(count).toBe(1);
+  });
+
+  test('a binding called with wrong types fails fast', async ({ app }) => {
+    // player.UserVolume is an int.  Passing a float makes the backend
+    // log "error parsing arguments" and never fire the callback; without
+    // a timeout the promise never settles and the spec hangs until the
+    // suite gives up.
+    const failure = await app.evaluate(async () => {
+      try {
+        await window.__yjEvents.call(
+          'player.Player.SetVolume',
+          [0.42],
+          2_000,
+        );
+
+        return 'settled';
+      } catch (err) {
+        return (err as Error).message;
+      }
+    });
+
+    expect(failure).toContain('did not settle');
+  });
+
+  test('the control surface is mounted and seeded', async ({ testctl }) => {
+    const health = await testctl.health();
+
+    expect(health.ok).toBe(true);
+    expect(health.libraries.length).toBeGreaterThan(0);
+    expect(health.counts.tracks).toBeGreaterThan(0);
+  });
+});