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>

bitchat/ViewModels/ChatMediaTransferCoordinator.swift

@@ -25,7 +25,6 @@ protocol ChatMediaTransferContext: AnyObject {
     func currentPublicSender() -> (name: String, peerID: PeerID)
 
     // MARK: Message state
-    var privateChats: [PeerID: [BitchatMessage]] { get }
     /// Appends a private message via the single-writer store intent.
     @discardableResult
     func appendPrivateMessage(_ message: BitchatMessage, to peerID: PeerID) -> Bool
@@ -52,7 +51,7 @@ protocol ChatMediaTransferContext: AnyObject {
 extension ChatViewModel: ChatMediaTransferContext {
     // `canSendMediaInCurrentContext`, `selectedPrivateChatPeer`, `nickname`,
     // `myPeerID`, `activeChannel`, `nicknameForPeer(_:)`,
-    // `currentPublicSender()`, `privateChats`,
+    // `currentPublicSender()`,
     // `appendPublicMessage(_:to:)`, `removeMessage(withID:cleanupFile:)`,
     // `addSystemMessage(_:)`, `notifyUIChanged()`,
     // `updateMessageDeliveryStatus(_:status:)`, `normalizedContentKey(_:)`,