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/ChatViewModelTests.swift

@@ -136,7 +136,7 @@ struct ChatViewModelIdentityTests {
             senderPeerID: oldPeerID,
             mentions: nil
         )
-        viewModel.privateChats[oldPeerID] = [existingMessage]
+        viewModel.seedPrivateChat([existingMessage], for: oldPeerID)
         viewModel.startPrivateChat(with: oldPeerID)
 
         #expect(viewModel.selectedPrivateChatPeer == oldPeerID)
@@ -345,8 +345,8 @@ struct ChatViewModelServiceLifecycleTests {
             mentions: nil
         )
 
-        viewModel.privateChats[peerID] = [message]
-        viewModel.unreadPrivateMessages.insert(peerID)
+        viewModel.seedPrivateChat([message], for: peerID)
+        viewModel.markPrivateChatUnread(peerID)
         viewModel.selectedPrivateChatPeer = peerID
 
         viewModel.handleDidBecomeActive()
@@ -497,7 +497,7 @@ struct ChatViewModelNoisePayloadTests {
             mentions: nil,
             deliveryStatus: .sent
         )
-        viewModel.privateChats[peerID] = [message]
+        viewModel.seedPrivateChat([message], for: peerID)
 
         viewModel.didReceiveNoisePayload(
             from: peerID,
@@ -535,7 +535,7 @@ struct ChatViewModelNoisePayloadTests {
             mentions: nil,
             deliveryStatus: .sent
         )
-        viewModel.privateChats[peerID] = [message]
+        viewModel.seedPrivateChat([message], for: peerID)
 
         viewModel.didReceiveNoisePayload(
             from: peerID,
@@ -771,7 +771,7 @@ struct ChatViewModelPeerTests {
     func didUpdatePeerList_removesStaleUnreadPeerWithoutMessages() async {
         let (viewModel, _) = makeTestableViewModel()
         let stalePeer = PeerID(str: "00000000000000a2")
-        viewModel.unreadPrivateMessages = [stalePeer]
+        viewModel.markPrivateChatUnread(stalePeer)
 
         viewModel.didUpdatePeerList([])
 
@@ -798,8 +798,8 @@ struct ChatViewModelPeerTests {
             senderPeerID: stablePeer,
             mentions: nil
         )
-        viewModel.privateChats[stablePeer] = [message]
-        viewModel.unreadPrivateMessages = [stablePeer]
+        viewModel.seedPrivateChat([message], for: stablePeer)
+        viewModel.markPrivateChatUnread(stablePeer)
 
         viewModel.didUpdatePeerList([])
         try? await Task.sleep(nanoseconds: 100_000_000)
@@ -876,33 +876,32 @@ struct ChatViewModelPrivateChatSelectionTests {
         let older = Date().addingTimeInterval(-120)
         let newer = Date().addingTimeInterval(-30)
 
-        viewModel.privateChats = [
-            peerA: [
-                BitchatMessage(
-                    id: "a-1",
-                    sender: "A",
-                    content: "Old",
-                    timestamp: older,
-                    isRelay: false,
-                    isPrivate: true,
-                    recipientNickname: "Me",
-                    senderPeerID: peerA
-                )
-            ],
-            peerB: [
-                BitchatMessage(
-                    id: "b-1",
-                    sender: "B",
-                    content: "New",
-                    timestamp: newer,
-                    isRelay: false,
-                    isPrivate: true,
-                    recipientNickname: "Me",
-                    senderPeerID: peerB
-                )
-            ]
-        ]
-        viewModel.unreadPrivateMessages = [peerA, peerB]
+        viewModel.seedPrivateChat([
+            BitchatMessage(
+                id: "a-1",
+                sender: "A",
+                content: "Old",
+                timestamp: older,
+                isRelay: false,
+                isPrivate: true,
+                recipientNickname: "Me",
+                senderPeerID: peerA
+            )
+        ], for: peerA)
+        viewModel.seedPrivateChat([
+            BitchatMessage(
+                id: "b-1",
+                sender: "B",
+                content: "New",
+                timestamp: newer,
+                isRelay: false,
+                isPrivate: true,
+                recipientNickname: "Me",
+                senderPeerID: peerB
+            )
+        ], for: peerB)
+        viewModel.markPrivateChatUnread(peerA)
+        viewModel.markPrivateChatUnread(peerB)
 
         viewModel.openMostRelevantPrivateChat()
 
@@ -918,32 +917,30 @@ struct ChatViewModelPrivateChatSelectionTests {
         let older = Date().addingTimeInterval(-200)
         let newer = Date().addingTimeInterval(-20)
 
-        viewModel.privateChats = [
-            peerA: [
-                BitchatMessage(
-                    id: "a-1",
-                    sender: "A",
-                    content: "Old",
-                    timestamp: older,
-                    isRelay: false,
-                    isPrivate: true,
-                    recipientNickname: "Me",
-                    senderPeerID: peerA
-                )
-            ],
-            peerB: [
-                BitchatMessage(
-                    id: "b-1",
-                    sender: "B",
-                    content: "New",
-                    timestamp: newer,
-                    isRelay: false,
-                    isPrivate: true,
-                    recipientNickname: "Me",
-                    senderPeerID: peerB
-                )
-            ]
-        ]
+        viewModel.seedPrivateChat([
+            BitchatMessage(
+                id: "a-1",
+                sender: "A",
+                content: "Old",
+                timestamp: older,
+                isRelay: false,
+                isPrivate: true,
+                recipientNickname: "Me",
+                senderPeerID: peerA
+            )
+        ], for: peerA)
+        viewModel.seedPrivateChat([
+            BitchatMessage(
+                id: "b-1",
+                sender: "B",
+                content: "New",
+                timestamp: newer,
+                isRelay: false,
+                isPrivate: true,
+                recipientNickname: "Me",
+                senderPeerID: peerB
+            )
+        ], for: peerB)
 
         viewModel.openMostRelevantPrivateChat()
 
@@ -1011,7 +1008,7 @@ struct ChatViewModelPanicTests {
                 isRelay: false
             )
         ]
-        viewModel.privateChats[PeerID(str: "PEER1")] = [
+        viewModel.seedPrivateChat([
             BitchatMessage(
                 id: "pm-1",
                 sender: "Peer",
@@ -1022,8 +1019,8 @@ struct ChatViewModelPanicTests {
                 recipientNickname: "Me",
                 senderPeerID: PeerID(str: "PEER1")
             )
-        ]
-        viewModel.unreadPrivateMessages.insert(PeerID(str: "PEER1"))
+        ], for: PeerID(str: "PEER1"))
+        viewModel.markPrivateChatUnread(PeerID(str: "PEER1"))
 
         viewModel.panicClearAllData()