Remove noise service exposure; single-owner selection state

Transport callers no longer reach the raw NoiseEncryptionService:
getNoiseService() is deleted in favor of narrow purpose-named Transport
methods (session public key, identity fingerprint, static/signing keys,
sign/verify, callback installation). VerificationService now reaches
crypto through the transport, so it can no longer pin a stale service
across a panic reset. myPeerID/myNickname become private(set); the
existing setNickname mutator is the sole nickname path.

ConversationStore is now the sole owner of private-chat selection:
PrivateChatManager.selectedPeer is a published read-only mirror, and
startChat/endChat mutate through the store intent. The bridge method
and its five call sites are deleted, removing a latent bug where a
stale manager selection pushed back into the store could resurrect a
just-removed conversation.

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

bitchatTests/ChatPeerIdentityCoordinatorContextTests.swift

@@ -66,7 +66,6 @@ private final class MockChatPeerIdentityContext: ChatPeerIdentityContext {
     private(set) var consolidatedPeers: [(peerID: PeerID, peerNickname: String)] = []
     private(set) var syncedReadReceiptPeers: [PeerID] = []
     private(set) var begunChatSessions: [PeerID] = []
-    private(set) var selectionStoreSyncCount = 0
     private(set) var markedReadPeers: [PeerID] = []
 
     @discardableResult
@@ -83,7 +82,6 @@ private final class MockChatPeerIdentityContext: ChatPeerIdentityContext {
         begunChatSessions.append(peerID)
     }
 
-    func synchronizeConversationSelectionStore() { selectionStoreSyncCount += 1 }
     func markPrivateMessagesAsRead(from peerID: PeerID) { markedReadPeers.append(peerID) }
 
     // Unified peer service
@@ -283,7 +281,6 @@ struct ChatPeerIdentityCoordinatorContextTests {
         #expect(context.storedFingerprints.map(\.fingerprint) == ["fp-alice"])
         #expect(context.selectedPrivateChatFingerprint == "fp-alice")
         #expect(context.begunChatSessions == [peerID])
-        #expect(context.selectionStoreSyncCount == 1)
         #expect(context.markedReadPeers == [peerID])
 
         // Established session: no second handshake.