Private groups: creator-managed encrypted group chat over the mesh

Small encrypted crews (hard cap 16) between public broadcast and 1:1 DMs:

Protocol
- MessageType.groupMessage = 0x25: broadcast packets with a cleartext
  16-byte group ID + epoch, ChaCha20-Poly1305 ciphertext (epoch bound as
  AEAD AAD), inner Ed25519 sender signature over
  "bitchat-group-msg-v1"|groupID|messageID|timestamp|content
- NoisePayloadType.groupInvite = 0x06 / .groupKeyUpdate = 0x07:
  creator-signed group state (key, epoch, roster) 1:1 over Noise; signature
  over "bitchat-group-v1"|groupID|epoch|key-hash|roster-hash and the Noise
  session peer must BE the creator
- SyncTypeFlags bit 10 (groupMessage): variable-length LE bitfield widens
  1 -> 2 bytes inside the length-prefixed REQUEST_SYNC TLV; old clients
  ignore unknown bits and answer with types they know
- PeerCapabilities.localSupported now advertises .groups

Storage
- GroupStore: symmetric keys in the keychain, roster/name/epoch as
  protected JSON in Application Support; wiped in panicClearAllData()

Behavior
- Non-members relay 0x25 like any broadcast but cannot read it; group
  messages join gossip-sync backfill with the public-message window
- Receivers drop wrong-epoch envelopes, bad sender signatures, and
  senders missing from the creator-signed roster
- Fire-and-flood delivery (no per-member acks in v1)

UI
- Groups open as chat windows through the private-chat sheet (virtual
  "group_" peer IDs); groups section in the people sheet; /group
  create/invite/remove/leave/list commands; invitees get a system message
  + notification and the group appears in their people sheet

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bitchatTests/ChatTransportEventCoordinatorContextTests.swift

@@ -156,6 +156,18 @@ private final class MockChatTransportEventContext: ChatTransportEventContext {
     func handleVerifyResponsePayload(from peerID: PeerID, payload: Data) {
         verifyResponsePayloads.append((peerID, payload))
     }
+
+    // Group payloads
+    private(set) var groupInvitePayloads: [(peerID: PeerID, payload: Data)] = []
+    private(set) var groupKeyUpdatePayloads: [(peerID: PeerID, payload: Data)] = []
+
+    func handleGroupInvitePayload(from peerID: PeerID, payload: Data) {
+        groupInvitePayloads.append((peerID, payload))
+    }
+
+    func handleGroupKeyUpdatePayload(from peerID: PeerID, payload: Data) {
+        groupKeyUpdatePayloads.append((peerID, payload))
+    }
 }
 
 // MARK: - Helpers