feat(events): EventBus + domain-модель статусов

- internal/model: единый источник статусов задачи (Status, TraceStatus,
  машина переходов), без зависимости от storage.
- internal/storage: совместимый мост (type Status = model.Status,
  re-export констант) — внешний код не меняется.
- internal/events: шина событий (fan-out, блокирующий Publish с гарантией
  порядка), события задач/трейсов, отдельная логовая шина + LogWriter,
  Publisher/NilPublisher для внедрения в Core/Worker.
- docs/ui-spec.md: спецификация десктопного UI (Fyne).

internal/events/bus_test.go

@@ -0,0 +1,119 @@
+package events
+
+import (
+	"testing"
+	"time"
+
+	"github.com/kamelion/ratatoskr-go/internal/model"
+)
+
+// receiveOne помогает получить одно событие с таймаутом.
+func receiveOne(t *testing.T, ch <-chan Event) Event {
+	t.Helper()
+	select {
+	case e := <-ch:
+		return e
+	case <-time.After(2 * time.Second):
+		t.Fatal("timeout waiting for event")
+		return nil
+	}
+}
+
+func TestBusPublishToSubscriber(t *testing.T) {
+	bus := New(10)
+	ch, unsub := bus.Subscribe()
+	defer unsub()
+
+	want := TaskStatusChanged{ID: 42, From: model.StatusReady, To: model.StatusRunning}
+	bus.Publish(want)
+
+	got := receiveOne(t, ch)
+	ev, ok := got.(TaskStatusChanged)
+	if !ok {
+		t.Fatalf("got %T, want TaskStatusChanged", got)
+	}
+	if ev.ID != 42 || ev.From != model.StatusReady || ev.To != model.StatusRunning {
+		t.Fatalf("unexpected event: %+v", ev)
+	}
+}
+
+func TestBusFanout(t *testing.T) {
+	bus := New(10)
+	ch1, unsub1 := bus.Subscribe()
+	defer unsub1()
+	ch2, unsub2 := bus.Subscribe()
+	defer unsub2()
+
+	e := LogLine{Level: "log", Text: "hello"}
+	bus.Publish(e)
+
+	if got := receiveOne(t, ch1); got != e {
+		t.Fatalf("subscriber 1 got %#v, want %#v", got, e)
+	}
+	if got := receiveOne(t, ch2); got != e {
+		t.Fatalf("subscriber 2 got %#v, want %#v", got, e)
+	}
+}
+
+func TestBusPreservesOrder(t *testing.T) {
+	bus := New(64)
+	ch, unsub := bus.Subscribe()
+	defer unsub()
+
+	const n = 25
+	for i := 0; i < n; i++ {
+		bus.Publish(TraceAppended{TaskID: int64(i)})
+	}
+	for i := 0; i < n; i++ {
+		ev := receiveOne(t, ch)
+		ta, ok := ev.(TraceAppended)
+		if !ok {
+			t.Fatalf("got %T, want TraceAppended", ev)
+		}
+		if ta.TaskID != int64(i) {
+			t.Fatalf("out of order: got %d, want %d", ta.TaskID, i)
+		}
+	}
+}
+
+func TestUnsubscribeStopsDelivery(t *testing.T) {
+	bus := New(10)
+	ch, unsub := bus.Subscribe()
+	bus.Publish(TaskCreated{ID: 1})
+	receiveOne(t, ch)
+
+	unsub()
+	if got := bus.SubscribersCount(); got != 0 {
+		t.Fatalf("SubscribersCount = %d, want 0", got)
+	}
+
+	// Убеждаемся, что Publish не блокируется навечно отписанным подписчиком.
+	bus.Publish(TaskCreated{ID: 2})
+
+	select {
+	case got := <-ch:
+		t.Fatalf("received %#v after unsubscribe", got)
+	case <-time.After(200 * time.Millisecond):
+	}
+}
+
+func TestBusSubscribersCount(t *testing.T) {
+	bus := New(10)
+	if got := bus.SubscribersCount(); got != 0 {
+		t.Fatalf("initial count = %d, want 0", got)
+	}
+	_, unsub1 := bus.Subscribe()
+	_, unsub2 := bus.Subscribe()
+	if got := bus.SubscribersCount(); got != 2 {
+		t.Fatalf("count = %d, want 2", got)
+	}
+	unsub1()
+	unsub2()
+	if got := bus.SubscribersCount(); got != 0 {
+		t.Fatalf("after unsub count = %d, want 0", got)
+	}
+}
+
+func TestNilPublisherNoOp(t *testing.T) {
+	NilPublisher{}.Publish(TaskCreated{ID: 1}) // must not panic
+}