Add capability bits to announce TLV

Announces now carry an optional capabilities TLV (0x05): a little-endian
bitfield with named bits for upcoming features (prekeys, wifiBulk,
gateway, groups, board, vouch, meshDiagnostics). Old clients skip the
unknown TLV; peers without it decode as nil so features can distinguish
"legacy peer" from "advertises nothing".

PeerCapabilities lives in BitFoundation with a minimal-length encoding
that preserves unknown bits for forward compatibility. Peer capabilities
are stored in the BLE peer registry on verified announce and exposed via
BLEService.peerCapabilities(_:). The local advertisement set is empty
until each feature ships its bit.

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

bitchatTests/Protocols/PacketsTests.swift

@@ -1,3 +1,4 @@
+import BitFoundation
 import Foundation
 import Testing
 
@@ -108,6 +109,42 @@ struct PacketsTests {
         #expect(decoded.directNeighbors == nil)
     }
 
+    @Test
+    func announcementPacketRoundTripsCapabilities() throws {
+        let capabilities: PeerCapabilities = [.prekeys, .board, .meshDiagnostics]
+        let packet = AnnouncementPacket(
+            nickname: "alice",
+            noisePublicKey: Data(repeating: 0x11, count: 32),
+            signingPublicKey: Data(repeating: 0x22, count: 32),
+            directNeighbors: nil,
+            capabilities: capabilities
+        )
+
+        let encoded = try #require(packet.encode())
+        let decoded = try #require(AnnouncementPacket.decode(from: encoded))
+        #expect(decoded.capabilities == capabilities)
+    }
+
+    @Test
+    func announcementPacketWithoutCapabilitiesDecodesNilAndUnknownBitsSurvive() throws {
+        let legacy = try #require(
+            AnnouncementPacket(
+                nickname: "alice",
+                noisePublicKey: Data(repeating: 0x11, count: 32),
+                signingPublicKey: Data(repeating: 0x22, count: 32),
+                directNeighbors: nil
+            ).encode()
+        )
+        // The TLV is emitted only when capabilities are set, so legacy peers
+        // (and this packet) decode as nil rather than empty.
+        #expect(try #require(AnnouncementPacket.decode(from: legacy)).capabilities == nil)
+
+        var withFutureBits = legacy
+        withFutureBits.append(makeTLV(type: 0x05, value: Data([0x80, 0x01])))
+        let decoded = try #require(AnnouncementPacket.decode(from: withFutureBits))
+        #expect(decoded.capabilities?.rawValue == 0x0180)
+    }
+
     @Test
     func privateMessagePacketRejectsUnknownTypeAndTruncation() {
         let unknownTLV = Data([0x7F, 0x01, 0x41])