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.

backend/player/player_test.go

@@ -4,13 +4,9 @@ import (
 	"log/slog"
 	"os"
 	"testing"
-)
 
-var testQueue = []string{
-	"../../test_data/music_library_test/other_music/03 PONPONPON.mp3",
-	"../../test_data/music_library_test/01 Some Chords.mp3",
-	"../../test_data/music_library_test/03 anything.mp3",
-}
+	"yellowjacket/internal/testfixtures"
+)
 
 func TestPlayer(t *testing.T) {
 	// This is an integration test that requires:
@@ -24,6 +20,17 @@ func TestPlayer(t *testing.T) {
 		)
 	}
 
+	// One track per supported container, so a decoder regression in
+	// any of the four shows up here rather than only in whichever
+	// format the fixtures happened to lead with.
+	m := testfixtures.Load(t)
+	testQueue := []string{
+		m.Case(t, testfixtures.CaseCoverDedup)[0],
+		m.Case(t, testfixtures.CaseFLACAlbum)[0],
+		m.Case(t, testfixtures.CaseOGGAlbum)[0],
+		m.Case(t, testfixtures.CaseWAVTracks)[0],
+	}
+
 	t.Logf("Starting test")
 
 	p := NewPlayer(slog.Default(), nil)