Cut private message path over to ConversationStore

All private-message mutations now flow through store intents:
coordinators, PrivateChatManager (its @Published dicts deleted - now
read-only views over the store), outbound sends, delivery status, and
chat migration. The O(1) store dedup replaces the full-scan duplicate
check; insertion order is maintained by the store so sanitizeChat's
re-sort is a documented no-op. Both bootstrapper Combine bridges and
the Task.yield store synchronization are deleted.

ChatViewModel.privateChats/unreadPrivateMessages become get-only derived
views (measured: naive rebuild equals a change-invalidated cache within
noise, so the simpler form stays). Feature models still read the legacy
store, fed by a coalescing LegacyConversationStoreBridge (one mirror
per burst, marked for step-5 deletion).

pipeline.privateIngest: 9.6k -> 14.7k msg/s (+53%).

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

bitchatTests/ChatMediaTransferCoordinatorContextTests.swift

@@ -42,6 +42,16 @@ private final class MockChatMediaTransferContext: ChatMediaTransferContext {
 
     // Message state
     var privateChats: [PeerID: [BitchatMessage]] = [:]
+
+    @discardableResult
+    func appendPrivateMessage(_ message: BitchatMessage, to peerID: PeerID) -> Bool {
+        var chat = privateChats[peerID] ?? []
+        guard !chat.contains(where: { $0.id == message.id }) else { return false }
+        chat.append(message)
+        privateChats[peerID] = chat
+        return true
+    }
+
     var meshTimeline: [BitchatMessage] = []
     private(set) var refreshedChannels: [ChannelID?] = []
     private(set) var trimCount = 0