Encrypt private media before BLE fragmentation (#1434)

Closes the last cleartext private-content path over BLE: private DM images/voice were sent as plaintext signed fileTransfer packets, TTL-relayed across the mesh, so every relay saw the full bytes. Now the complete BitchatFilePacket is encrypted as a single Noise AEAD message (inner type 0x20, matching Android) and the opaque ciphertext is fragmented. Adds an authenticated in-session capability proof (0x21 TLV: capabilities + Ed25519 key), TOFU-style downgrade pinning, a per-send consent dialog for the signed-cleartext fallback to legacy peers, and a cancellation/admission registry so cancel/delete cannot race a deferred cleartext send.

Android wire constants (0x20 / 0x21 / capability bit 8) confirmed shipping. The 256-fragment preflight cap applies only to the directed fileTransfer migration fallback; encrypted media to capable peers uses the full receiver ceiling.

Rebased over #1428/#1349: identity reads go through BLELocalIdentityStateStore; the session-bound authenticated signing-key check and the announce-path TOFU pin are kept as complementary checks. Full local suite green (1744+197 tests).

localPackages/BitFoundation/Sources/BitFoundation/PeerCapabilities.swift

@@ -24,6 +24,10 @@ public struct PeerCapabilities: OptionSet, Equatable, Hashable, Sendable {
     /// (uplink/downlink carriers for mesh-only peers). Advertised alongside
     /// a `bridgeGeohash` TLV carrying the rendezvous cell.
     public static let bridge = PeerCapabilities(rawValue: 1 << 7)
+    /// Finalized direct-message media encrypted as Noise payload `0x20`
+    /// before outer BLE fragmentation. Peers that omit this bit require the
+    /// signed directed raw-file migration fallback.
+    public static let privateMedia = PeerCapabilities(rawValue: 1 << 8)
 
     /// Minimal little-endian byte encoding; always at least one byte so an
     /// empty set is distinguishable from an absent TLV.