Gate connected-link trust on a secure session; deny forged direct announces the connected shortcut

Residual gap after the rotation-heal containment (#1401): "verified
direct" announces prove the signature but not directness — TTL is
unsigned — so a malicious connected peer can replay a victim's fresh
announce with its TTL restored. When the victim has no live link, the
replayer's link rebinds to the victim's ID and reads as "connected",
and MessageRouter's connected fast-path then trusts it outright: every
DM stalls on a Noise handshake the replayer can never complete and is
silently lost while showing "sent".

Router-level trust gate (no wire change):
- Transport gains canDeliverSecurely(to:) — BLE answers with an
  established Noise session; Nostr keeps its prompt-delivery predicate;
  the protocol default forwards to canDeliverPromptly for transports
  without a forgeable link layer.
- MessageRouter.sendPrivate only trusts a connected link outright when
  it can deliver securely; otherwise it still sends (kicking the
  handshake on a genuine link) but retains a copy and hands a sealed
  copy to couriers, like the reachable path. flushOutbox gets the same
  gate so a flush over an insecure link resends instead of dropping the
  retained copy.

Presence hardening (defense in depth): a "direct" announce arriving on
a link already bound to a different peer no longer shortcuts the
claimed peer into "connected" — only real link state does. Genuine
first-contact and direct announces are unaffected; only the ambiguous
heal path loses the forgeable shortcut.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

bitchatTests/BLEServiceCoreTests.swift

@@ -253,6 +253,57 @@ struct BLEServiceCoreTests {
         #expect(ble.currentPeerSnapshots().map(\.peerID).contains(attackerPeerID))
     }
 
+    @Test
+    func replayedDirectAnnounceForAbsentPeerDoesNotForgeConnectivity() async throws {
+        // Residual heal-path gap: the victim has NO live link, so the
+        // identity-owns-a-link containment cannot refuse the rebind. The
+        // replay may steal the link binding, but it must not mark the absent
+        // victim connected (TTL — the directness signal — is unsigned), and
+        // it can never produce a deliverable secure session.
+        let ble = makeService()
+        let attackerPeerID = PeerID(str: "1122334455667788")
+        let attackerLink = "central-attacker-absent-victim"
+        ble._test_seedConnectedPeer(attackerPeerID, nickname: "attacker")
+        ble._test_bindCentral(attackerLink, to: attackerPeerID)
+
+        // The absent victim's fresh signed announce, replayed on the
+        // attacker's bound link with its direct TTL restored.
+        let victimSigner = NoiseEncryptionService(keychain: MockKeychain())
+        let announcement = AnnouncementPacket(
+            nickname: "victim",
+            noisePublicKey: victimSigner.getStaticPublicKeyData(),
+            signingPublicKey: victimSigner.getSigningPublicKeyData(),
+            directNeighbors: nil
+        )
+        let payload = try #require(announcement.encode(), "Failed to encode announcement")
+        let victimPeerID = PeerID(publicKey: announcement.noisePublicKey)
+        let unsigned = BitchatPacket(
+            type: MessageType.announce.rawValue,
+            senderID: Data(hexString: victimPeerID.id) ?? Data(),
+            recipientID: nil,
+            timestamp: UInt64(Date().timeIntervalSince1970 * 1000),
+            payload: payload,
+            signature: nil,
+            ttl: 7
+        )
+        let packet = try #require(victimSigner.signPacket(unsigned), "Failed to sign announce packet")
+
+        #expect(ble._test_recordIngressIfNew(packet: packet, linkID: attackerLink))
+        ble._test_handlePacket(packet, fromPeerID: victimPeerID, preseedPeer: false)
+
+        // The victim becomes known (verified announce) …
+        let known = await TestHelpers.waitUntil(
+            { ble.currentPeerSnapshots().map(\.peerID).contains(victimPeerID) },
+            timeout: TestConstants.longTimeout
+        )
+        #expect(known)
+        // … but the forged direct TTL must not mark it connected, and the
+        // link can never deliver securely, so MessageRouter falls through to
+        // retain + courier instead of trusting the stolen link outright.
+        #expect(!ble.isPeerConnected(victimPeerID))
+        #expect(!ble.canDeliverSecurely(to: victimPeerID))
+    }
+
     @Test
     func ingressRejectsSelfLoopbackBeforeSpoofChecks() async throws {
         let ble = makeService()