Fix critical thread-safety crash in PeerID fragment handler

CRITICAL: The PeerID version of _handleFragment was accessing
incomingFragments dictionary without collectionsQueue synchronization,
causing crashes when multiple BLE threads processed fragments concurrently.

Crash stack trace pointed to line 3431 (dictionary subscript) with:
'doesNotRecognizeSelector' - classic concurrent mutation crash.

Fix:
- Wrapped ALL incomingFragments/fragmentMetadata access in
  collectionsQueue.sync(flags: .barrier)
- Matches the thread-safe pattern used in String version
- Separate cleanup into its own barrier block after reassembly
- Prevents concurrent dictionary mutations from multiple BLE threads

This is the same pattern as the String version (line 1128) which didn't crash.

bitchat/Services/BLEService.swift

@@ -3417,8 +3417,14 @@ extension BLEService {
         // Sanity checks - add reasonable upper bound on total to prevent DoS
         guard total > 0 && total <= 10000 && index >= 0 && index < total else { return }
 
-        // Store fragment
+        // Compute fragment key for this assembly
         let key = FragmentKey(sender: senderU64, id: fragU64)
+
+        // Critical section: Store fragment and check completion status
+        var shouldReassemble: Bool = false
+        var fragmentsToReassemble: [Int: Data]? = nil
+
+        collectionsQueue.sync(flags: .barrier) {
             if incomingFragments[key] == nil {
                 // Cap in-flight assemblies to prevent memory/battery blowups
                 if incomingFragments.count >= maxInFlightAssemblies {
@@ -3450,15 +3456,26 @@ extension BLEService {
                 )
                 incomingFragments.removeValue(forKey: key)
                 fragmentMetadata.removeValue(forKey: key)
+                shouldReassemble = false
+                fragmentsToReassemble = nil
                 return
             }
 
             incomingFragments[key]?[index] = Data(fragmentData)
 
             // Check if complete
-        if let fragments = incomingFragments[key],
+            if let fragments = incomingFragments[key], fragments.count == total {
-           fragments.count == total {
+                shouldReassemble = true
-            // Reassemble
+                fragmentsToReassemble = fragments
+            } else {
+                shouldReassemble = false
+                fragmentsToReassemble = nil
+            }
+        }
+
+        // Heavy work outside lock: reassemble and decode
+        guard shouldReassemble, let fragments = fragmentsToReassemble else { return }
+
         var reassembled = Data()
         for i in 0..<total {
             if let fragment = fragments[i] {
@@ -3473,7 +3490,8 @@ extension BLEService {
             SecureLogger.error("❌ Failed to decode reassembled packet (type=\(originalType), total=\(total))", category: .session)
         }
 
-            // Cleanup
+        // Critical section: Cleanup completed assembly
+        collectionsQueue.sync(flags: .barrier) {
             incomingFragments.removeValue(forKey: key)
             fragmentMetadata.removeValue(forKey: key)
         }