fix(build): keep the index tools free of the Wails application

The v3 migration put application.Get() in backend/events and a
ServiceStartup hook in backend/explore, both of which cmd/indexbuild
reaches. v3's application package is GTK/WebKit bindings on Linux, so
the index-artifact job — a plain golang container with CGO_ENABLED=0,
on the stated grounds that neither command imports the app — stopped
compiling with "undefined: pointer". That job owns the ~205 GB dump
checkpoint, so it is the worst place to learn this.

Both are behind the indexbuild tag now: the one app.Event.Emit lives in
runtime_wails.go, runtime_indexbuild.go answers ErrNoRuntime (what the
app itself returns before Run, so Deliver's callers need no second
path), and explore's ServiceStartup moves to its own tagged file.

TestIndexToolsDoNotImportWails walks `go list -deps -tags indexbuild`
so the claim the workflow makes is checked rather than assumed.

backend/events/emit.go

@@ -4,8 +4,6 @@ import (
 	"context"
 	"errors"
 	"log/slog"
-
-	"github.com/wailsapp/wails/v3/pkg/application"
 )
 
 // ErrNoRuntime is returned by Deliver when there is neither a test Sink
@@ -82,16 +80,7 @@ func Deliver(ctx context.Context, name string, data ...any) error {
 		}
 	}
 
-	// application.Get() returns nil when no app is running — under test,
-	// before Run, and after shutdown.  It does not terminate the
-	// process, which is what the v2 probe of the private "events"
-	// context key existed to avoid.
-	app := application.Get()
-	if app == nil {
-		return ErrNoRuntime
-	}
-
-	app.Event.Emit(name, data...)
-
-	return nil
+	// emitRuntime is the only place the Wails application is touched,
+	// and it is behind a build tag: see runtime_wails.go.
+	return emitRuntime(name, data...)
 }