Geohash bulletin board: persistent signed notices over mesh sync (#1379)

* Add geohash bulletin board: persistent signed notices over mesh sync

New MessageType 0x23 carries TLV-encoded board posts and tombstones,
self-signed with the author's Ed25519 key ("bitchat-board-v1" /
"bitchat-board-del-v1" domains) so notices verify without the author
present. BoardStore persists raw signed packets under Application
Support/board/ (200 posts, 5 per author, oldest evicted; expiry sweep;
tombstones retained until the deleted post's original expiry) and is
wiped on panic.

Board packets join gossip sync as bit 8 of the existing variable-length
types bitfield (a second byte old decoders already accept and ignore),
with a 60s round and its own capacity, served straight from the board
store so retention has one owner. Posts relay like broadcasts; urgent
posts get the announce-class TTL cap.

UI: a pin button in the header opens the board for the current channel
(geohash board, or mesh-local board), with urgent-pinned newest-first
listing, compose with urgent toggle and 1/3/7-day expiry, and
swipe-delete on own posts. Geohash posts also publish one-way as
Nostr kind-1 location notes when relays are reachable.

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

* Board: bound orphan tombstones and reject future-dated posts at ingest

Two hardening fixes from Codex review of the geohash bulletin board:

- Orphan tombstones (P1): retention was derived solely from the
  sender-chosen deletedAt, so self-signed tombstones for unseen post IDs
  with far-future deletedAt persisted and re-entered sync unboundedly.
  Retention is now also clamped to receive time (now + 7d + 1h skew --
  no post can outlive that), and orphans are capped at 100 globally and
  5 per author key with oldest-received evicted first. Matched
  tombstones and disk restores keep their existing behavior.

- Future-dated posts (P2): ingest only checked expiresAt > now, letting
  posts dated years ahead sort above honest posts and squat the 200
  global slots without ever pruning. The single ingest chokepoint
  (radio, sync, and disk restore all funnel through it) now rejects
  createdAt > now + 1h skew and expiresAt > now + 7d + 1h skew; the
  decoder's span rule is unchanged.

Adds tests for the skew boundary, far-future expiry, receive-time
tombstone clamping, orphan caps/eviction, and matched-tombstone
exemption.

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

---------

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

localPackages/BitFoundation/Sources/BitFoundation/MessageType.swift

@@ -24,7 +24,8 @@ public enum MessageType: UInt8 {
     // Fragmentation (simplified)
     case fragment = 0x20        // Single fragment type for large messages
     case fileTransfer = 0x22    // Binary file/audio/image payloads
-    
+    case boardPost = 0x23       // Signed geohash bulletin-board post or tombstone
+
     public var description: String {
         switch self {
         case .announce: return "announce"
@@ -36,6 +37,7 @@ public enum MessageType: UInt8 {
         case .noiseEncrypted: return "noiseEncrypted"
         case .fragment: return "fragment"
         case .fileTransfer: return "fileTransfer"
+        case .boardPost: return "boardPost"
         }
     }
 }