Verify Nostr inbound signatures once, off the main actor

Every inbound relay event was Schnorr-verified TWICE, both times on the
main actor: once in NostrRelayManager.handleParsedMessage and again in
each NostrInboundPipeline / GeoPresenceTracker handler. Each
verification re-serializes the event JSON, hashes it, and runs a
secp256k1 Schnorr verify, so busy geohashes paid double crypto on the
UI thread. Geohash gift-wrap NIP-17 decryption (two ECDH+ChaCha layers)
also ran on the main actor.

Changes:
- NostrRelayManager now owns a serial off-main inbound pipeline
  (AsyncStream + single consumer task): frames are parsed and verified
  in arrival order off the main actor, preserving per-subscription
  delivery order. This is the single verification point for the whole
  inbound path; downstream handlers only ever see verified events.
- Dedup stays two-phase and unpoisonable: a cheap main-actor duplicate
  LOOKUP runs before verification (duplicate fan-in from several relays
  never pays for crypto — previously every duplicate was verified), and
  events are RECORDED as seen only after the signature verifies, so a
  forged-signature copy can never suppress the genuine event.
- NostrInboundPipeline and GeoPresenceTracker drop their redundant
  re-verification; geohash gift-wrap decryption moves off the main
  actor following the existing account-mailbox Task.detached pattern
  (atomic main-actor check-and-record, then decrypt off-main, then hop
  back for state updates).
- Tests: relay-level coverage for tampered gift wraps and for in-order
  delivery of back-to-back frames; pipeline-level tampered-signature
  tests move to the relay boundary where the invariant now lives; the
  mock relay connection queues frames emitted before receive re-arms.

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

bitchatTests/ChatViewModelExtensionsTests.swift

@@ -352,31 +352,11 @@ struct ChatViewModelNostrExtensionTests {
         #expect(!viewModel.messages.contains { $0.content == "Blocked" })
     }
 
-    @Test @MainActor
-    func handleNostrEvent_rejectsInvalidSignature() async throws {
-        let (viewModel, _) = makeTestableViewModel()
-        let geohash = "u4pruydq"
-        let identity = try NostrIdentity.generate()
-
-        viewModel.switchLocationChannel(to: .location(GeohashChannel(level: .city, geohash: geohash)))
-
-        let event = NostrEvent(
-            pubkey: identity.publicKeyHex,
-            createdAt: Date(),
-            kind: .ephemeralEvent,
-            tags: [["g", geohash]],
-            content: "Valid"
-        )
-        var signed = try event.sign(with: identity.schnorrSigningKey())
-        signed.id = "deadbeef"
-
-        viewModel.handleNostrEvent(signed)
-
-        try? await Task.sleep(nanoseconds: 100_000_000)
-        viewModel.publicMessagePipeline.flushIfNeeded()
-
-        #expect(!viewModel.messages.contains { $0.content == "Tampered" })
-    }
+    // NOTE: Tampered-signature rejection is enforced once, off the main
+    // actor, at the relay boundary (events only reach the inbound pipeline
+    // after verification) — see NostrRelayManagerTests
+    // `test_receiveEvent_invalidSignatureDoesNotPoisonDuplicateCache` and
+    // `test_receiveGiftWrap_tamperedSignatureIsDroppedAndDoesNotPoisonDedup`.
 
     @Test @MainActor
     func subscribeGiftWrap_rejectsOversizedEmbeddedPacket() async throws {
@@ -565,9 +545,14 @@ struct ChatViewModelNostrExtensionTests {
 
         viewModel.handleGiftWrap(giftWrap, id: recipient)
 
-        try? await Task.sleep(nanoseconds: 50_000_000)
+        // Gift-wrap decryption runs off the main actor; wait for the ack
+        // (sent even for blocked senders) to know processing finished.
+        let didAck = await TestHelpers.waitUntil(
+            { viewModel.sentGeoDeliveryAcks.contains(messageID) },
+            timeout: 5.0
+        )
+        #expect(didAck)
         #expect(viewModel.privateChats[convKey] == nil)
-        #expect(viewModel.sentGeoDeliveryAcks.contains(messageID))
     }
 
     @Test @MainActor