Cut views over to ConversationStore; delete legacy store, bridge, resolver

Feature models observe per-conversation objects directly: PublicChatModel
forwards the active Conversation's objectWillChange, PrivateInboxModel
republishes only for the selected peer's conversation - background
appends no longer invalidate foreground views. LegacyConversationStore,
the coalescing bridge, and IdentityResolver are deleted (resolver
canonicalization proved display-invisible: nothing enumerates direct
conversations, lookups are by exact peer ID, and raw keying is strictly
more robust - documented as a design deviation). Selection state moves
into the store. ChatViewModel.messages/privateChats survive as derived
read views for coordinators that genuinely need them; hot paths use a
new store-direct privateMessages(for:) witness.

Final numbers vs pre-migration baselines:
pipeline.privateIngest 9.7k -> 24.0k msg/s (2.5x)
pipeline.publicIngest  6.8k -> 13.7k msg/s (2.0x)
delivery updates       38k  -> 117-133k/s

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

bitchatTests/ChatTransportEventCoordinatorContextTests.swift

@@ -28,6 +28,10 @@ private final class MockChatTransportEventContext: ChatTransportEventContext {
     var nickname = "me"
     var myPeerID = PeerID(str: "0011223344556677")
     var privateChats: [PeerID: [BitchatMessage]] = [:]
+
+    func privateMessages(for peerID: PeerID) -> [BitchatMessage] {
+        privateChats[peerID] ?? []
+    }
     var unreadPrivateMessages: Set<PeerID> = []
     var selectedPrivateChatPeer: PeerID?
     private(set) var unmarkedReadReceiptBatches: [[String]] = []