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/app.go

@@ -33,6 +33,7 @@ import (
 	"yellowjacket/backend/queue"
 	"yellowjacket/backend/system"
 	"yellowjacket/backend/tagwriter"
+	"yellowjacket/backend/testctl"
 )
 
 // YellowJacketApp is the main application struct for Wails.
@@ -131,6 +132,17 @@ func NewYellowJacketApp(
 		yjApp.assetHandler.RegisterHandler("/artist-images/", artistImgHandler)
 	}
 
+	// Dev-only /__test/ control surface: the residue of harness work the
+	// browser cannot reach (snapshot/restore the DB mid-run, force a
+	// backend event).  Compiled out of non-dev builds entirely, and even
+	// in a dev build it registers nothing unless YJ_TESTCTL=1.  The
+	// context is read lazily because it only exists after OnStartup.
+	testctl.Register(yjApp.assetHandler, testctl.Deps{
+		Logger:  logger,
+		DB:      yjApp.database,
+		Context: func() context.Context { return yjApp.appContext },
+	})
+
 	// create playlist service
 	yjApp.playlist = playlist.NewService(
 		yjApp.logger, yjApp.database, yjApp.appConfig,
@@ -290,7 +302,7 @@ func (yj *YellowJacketApp) initDownloadRuntime(ctx context.Context) {
 	yj.wanted.SetInterval(cfg.WantedInterval())
 	yj.wanted.SetBatch(cfg.WantedBatch)
 	yj.wanted.SetOnChange(func() {
-		wailsruntime.EventsEmit(ctx, events.RequestsChanged)
+		events.Emit(ctx, events.RequestsChanged)
 	})
 	yj.wanted.Start(ctx)
 }