feat(workspace): add real-time live cursor and presence tracking

- Support ephemeral cursor_moved and client_left events in docroom Hub
- Keep active in-memory cursor presence map per document room
- Synchronize current presence state to newly connected WebSocket clients
- Allow read-only viewers to share cursor positions without document mutation rights
- Throttle frontend cursor move emissions to 100ms with instant blur cleanup
- Render active collaborator presence avatar pills in document header
- Display user-colored block indicator and non-destructive caret overlay with name tag

internal/ws/docroom_test.go

@@ -0,0 +1,85 @@
+package ws
+
+import (
+	"testing"
+	"time"
+)
+
+func TestDocRoomCursorPresenceAndEchoProtection(t *testing.T) {
+	hub := NewHub()
+	room := hub.GetOrCreateDocRoom("doc_test_presence")
+
+	c1 := &DocWSClient{
+		hub:      hub,
+		room:     room,
+		userID:   "user_1",
+		clientID: "cli_1",
+		canWrite: true,
+		send:     make(chan []byte, 10),
+	}
+	c2 := &DocWSClient{
+		hub:      hub,
+		room:     room,
+		userID:   "user_2",
+		clientID: "cli_2",
+		canWrite: false, // read-only viewer
+		send:     make(chan []byte, 10),
+	}
+
+	room.mu.Lock()
+	room.wsClients[c1] = true
+	room.wsClients[c2] = true
+	room.mu.Unlock()
+
+	// 1. Send cursor_moved from c1
+	selStart := 5
+	selEnd := 5
+	room.HandleEvent(DocSyncEvent{
+		Type:           "cursor_moved",
+		DocID:          "doc_test_presence",
+		ClientID:       "cli_1",
+		UserID:         "user_1",
+		UserName:       "Alice",
+		UserColor:      "#3b82f6",
+		BlockID:        "blk_123",
+		SelectionStart: &selStart,
+		SelectionEnd:   &selEnd,
+	}, c1)
+
+	// c1 should NOT have received its own echo
+	select {
+	case <-c1.send:
+		t.Fatal("c1 received echo of its own cursor_moved event!")
+	default:
+	}
+
+	// c2 SHOULD receive the event
+	select {
+	case msg := <-c2.send:
+		if len(msg) == 0 {
+			t.Fatal("empty message received by c2")
+		}
+	case <-time.After(500 * time.Millisecond):
+		t.Fatal("timed out waiting for cursor_moved event on c2")
+	}
+
+	// Room should have registered c1's presence
+	room.mu.Lock()
+	cp, exists := room.activeCursors["cli_1"]
+	room.mu.Unlock()
+
+	if !exists || cp == nil {
+		t.Fatal("expected active cursor for cli_1 in room")
+	}
+	if cp.UserName != "Alice" || cp.BlockID != "blk_123" {
+		t.Fatalf("unexpected cursor data: %+v", cp)
+	}
+
+	// Ephemeral cursor events must NOT mark the room dirty for disk flush
+	room.mu.Lock()
+	dirty := room.dirty
+	room.mu.Unlock()
+	if dirty {
+		t.Fatal("cursor_moved should not mark room dirty")
+	}
+}