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>

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"
         }
     }
 }