Expand store-and-forward: open couriers, spray-and-wait, persistent outbox, 6h public history (#1372)

Store-and-forward previously delivered to an out-of-range peer only if a
mutual favorite happened to be connected at send time and later met the
recipient directly, and everything except courier envelopes died with the
app process. This closes those gaps end to end:

- Persist the MessageRouter outbox to disk, sealed with a ChaChaPoly key
  held only in the Keychain (no plaintext at rest); queued private
  messages now survive an app kill and flush on next launch.
- Deposit retry: queued messages are re-deposited whenever a new eligible
  courier connects, tracked per message so the same courier is never
  double-burned, until 3 distinct couriers carry it or it expires.
- Tiered open couriering: signature-verified strangers can now carry mail
  (2 envelopes/depositor into a 20-slot pool) alongside mutual favorites
  (5 each); overflow evicts verified-tier mail before favorites'.
- Spray-and-wait: envelopes carry a copy budget (4, capped 8, new TLV,
  wire-compatible with old clients); couriers split half their remaining
  budget with each newly encountered courier so mail diffuses through a
  moving crowd.
- Remote handover: a verified relayed announce now floods a copy toward
  the multi-hop recipient (directed-relay treatment, 10-min per-envelope
  cooldown) while the carried original stays put for a direct encounter.
- Public history: gossip-sync window for whole public messages widened
  from 15 min to 6 h, matched on the receive-acceptance side, and the
  message store persists to disk so devices bridge partitions and
  restarts ("town crier").
- Privacy-safe local delivery counters (bare tallies, log-only) so the
  store-and-forward stack is measurable on-device.
- Panic wipe now also clears the sealed outbox, gossip archive, and
  counters.
- Rewrite WHITEPAPER.md to describe the app as implemented (Noise XX/X,
  actual flood control, courier system, gossip sync, Nostr path); the old
  document described a bloom filter, three fragment types, and a
  MessageRetryService that don't exist.

1037 macOS tests pass (17 new); iOS builds.

Co-authored-by: jack <jackjackbits@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

bitchat/Services/TransportConfig.swift

@@ -262,7 +262,14 @@ enum TransportConfig {
     static let syncSeenCapacity: Int = 1000
     static let syncGCSMaxBytes: Int = 400
     static let syncGCSTargetFpr: Double = 0.01
+    // Fragments and file transfers keep the short window; whole public
+    // messages get hours so a phone walking between partitions carries the
+    // room's recent history with it (see syncPublicMessageMaxAgeSeconds).
     static let syncMaxMessageAgeSeconds: TimeInterval = 900
+    // How far back public broadcast messages stay sync-able. Must not exceed
+    // the receive-side acceptance window (BLEPublicMessagePolicy uses this
+    // same constant) or served packets would be dropped as stale.
+    static let syncPublicMessageMaxAgeSeconds: TimeInterval = 6 * 60 * 60
     static let syncMaintenanceIntervalSeconds: TimeInterval = 30.0
     static let syncStalePeerCleanupIntervalSeconds: TimeInterval = 60.0
     static let syncStalePeerTimeoutSeconds: TimeInterval = 60.0
@@ -273,4 +280,13 @@ enum TransportConfig {
     static let syncMessageIntervalSeconds: TimeInterval = 15.0
     static let syncResponseRateLimitMaxResponses: Int = 8
     static let syncResponseRateLimitWindowSeconds: TimeInterval = 30.0
+
+    // Courier store-and-forward
+    // Initial spray-and-wait budget per deposited envelope: each courier may
+    // hand half its remaining copies to another courier on encounter, so a
+    // message diffuses through a moving crowd instead of riding one person.
+    static let courierInitialCopies: UInt8 = 4
+    // Cooldown between speculative multi-hop handovers of the same envelope
+    // toward a recipient heard only via relayed announces.
+    static let courierRemoteHandoverCooldownSeconds: TimeInterval = 10 * 60
 }