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/log_test.go

@@ -0,0 +1,76 @@
+package events
+
+import (
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestLogWriterLines(t *testing.T) {
+	lbus := NewLogBus(16)
+	ch, unsub := lbus.Subscribe()
+	defer unsub()
+
+	w := NewLogWriter(lbus)
+	if _, err := w.Write([]byte("first line\nsecond line\n")); err != nil {
+		t.Fatalf("Write: %v", err)
+	}
+
+	first := receiveOne(t, ch)
+	ll, ok := first.(LogLine)
+	if !ok {
+		t.Fatalf("got %T, want LogLine", first)
+	}
+	if ll.Text != "first line" || ll.Level != "log" {
+		t.Fatalf("unexpected first line: %+v", ll)
+	}
+
+	second := receiveOne(t, ch)
+	if ll, ok := second.(LogLine); !ok || ll.Text != "second line" {
+		t.Fatalf("unexpected second line: %+v", second)
+	}
+}
+
+func TestLogWriterPartialLine(t *testing.T) {
+	lbus := NewLogBus(16)
+	ch, unsub := lbus.Subscribe()
+	defer unsub()
+
+	w := NewLogWriter(lbus)
+	if _, err := w.Write([]byte("partial")); err != nil {
+		t.Fatalf("Write: %v", err)
+	}
+	select {
+	case got := <-ch:
+		t.Fatalf("partial line should not be published yet, got %#v", got)
+	case <-time.After(150 * time.Millisecond):
+	}
+
+	if _, err := w.Write([]byte(" line\n")); err != nil {
+		t.Fatalf("Write: %v", err)
+	}
+	got := receiveOne(t, ch)
+	ll, ok := got.(LogLine)
+	if !ok || ll.Text != "partial line" {
+		t.Fatalf("unexpected: %#v", got)
+	}
+}
+
+func TestLogWriterMultiSplit(t *testing.T) {
+	lbus := NewLogBus(16)
+	ch, unsub := lbus.Subscribe()
+	defer unsub()
+
+	w := NewLogWriter(lbus)
+	if _, err := w.Write([]byte("line1\nline2\nline3\n")); err != nil {
+		t.Fatalf("Write: %v", err)
+	}
+	var texts []string
+	for i := 0; i < 3; i++ {
+		ev := receiveOne(t, ch)
+		texts = append(texts, ev.(LogLine).Text)
+	}
+	if strings.Join(texts, ",") != "line1,line2,line3" {
+		t.Fatalf("got %v", texts)
+	}
+}