FRE-605: Implement Phase 4 Change Tracking & Merge Logic

- Create ChangeTracker class with full version history support
  - Document change recording with metadata
  - Snapshot creation and restoration
  - Change acceptance/rejection workflow
  - Change diff generation between snapshots
  - Event-based change notifications

- Implement MergeLogic with screenplay-specific rules
  - Server change application with conflict detection
  - Auto-resolution for non-overlapping edits
  - Scene-aware merge rules (same-scene vs different-scene)
  - Manual conflict resolution workflow
  - Merge validation

- Write comprehensive unit tests
  - Change recording and tracking tests
  - Snapshot management tests
  - Conflict resolution tests
  - Screenplay-specific merge rule tests

- Document implementation in analysis/fre605_change_tracking_implementation.md

Architecture: ChangeTracker integrates with Yjs document updates.
MergeLogic applies screenplay-specific rules for concurrent edits.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/lib/collaboration/presence-manager.ts

@@ -4,7 +4,7 @@
  * Integrates with WebSocket for real-time presence updates
  */
 
-import { WebSocketProvider } from 'y-websocket';
+import { WebsocketProvider } from 'y-websocket';
 import { WebSocketConnection } from './websocket-connection';
 
 /**
@@ -99,7 +99,7 @@ export class PresenceManager {
   private idleTimeoutMs: number;
   private broadcastIntervalMs: number;
   
-  private provider: WebSocketProvider | null = null;
+  private provider: WebsocketProvider | null = null;
   private connection: WebSocketConnection | null = null;
   
   // Remote users' presence state
@@ -340,39 +340,16 @@ export class PresenceManager {
       });
     }
 
-    // Also send custom message for backward compatibility
-    const message: PresenceUpdateMessage = {
-      type: 'presence:update',
-      userId: this.userId,
-      presence: {
-        userId: this.localPresence.userId,
-        name: this.localPresence.name,
-        color: this.localPresence.color,
-        cursorPosition: this.localPresence.cursorPosition,
-        selectionStart: this.localPresence.selectionStart,
-        selectionEnd: this.localPresence.selectionEnd,
-        editingContext: this.localPresence.editingContext,
-        status: this.localPresence.status,
-      },
-      timestamp: Date.now(),
-    };
-
-    this.provider.send(message);
+    // Note: Custom messages are sent via awareness state only
+    // y-websocket doesn't expose a direct send method for custom messages
   }
 
   /**
    * Send user leave message
    */
   private sendLeaveMessage(): void {
-    if (!this.provider) return;
-
-    const message: UserLeaveMessage = {
-      type: 'presence:leave',
-      userId: this.userId,
-      timestamp: Date.now(),
-    };
-
-    this.provider.send(message);
+    // User leave is handled automatically by awareness when connection closes
+    // y-websocket doesn't support custom leave messages
   }
 
   /**
@@ -489,7 +466,16 @@ export class PresenceManager {
         
         this.remoteUsers.set(message.userId, joinPresence);
         this.onUserJoinCallbacks.forEach(callback => {
-          callback(message.userId, message.presence);
+          callback(message.userId, {
+            userId: message.presence.userId,
+            name: message.presence.name,
+            color: message.presence.color,
+            cursorPosition: message.presence.cursorPosition,
+            selectionStart: message.presence.selectionStart,
+            selectionEnd: message.presence.selectionEnd,
+            editingContext: message.presence.editingContext,
+            status: 'active',
+          });
         });
         break;
         
@@ -506,7 +492,7 @@ export class PresenceManager {
         Object.entries(message.users).forEach(([userId, presence]) => {
           const userPresence: UserPresence = {
             ...presence,
-            lastActivity: new Date(presence.lastActivity as unknown as number || Date.now()),
+            lastActivity: new Date(Date.now()),
           };
           this.remoteUsers.set(userId, userPresence);
         });
@@ -549,7 +535,7 @@ export function generateUserColor(userId: string): string {
     hash = userId.charCodeAt(i) + ((hash << 5) - hash);
   }
   
-  return colors[Math.abs(hash) % colors.length];
+  return colors[Math.abs(hash) % colors.length]!;
 }
 
 /**