feat(mesh): MessageKey foundation and debug-dump RPC

Adds sender_pubkey + sender_seq fields to MeshMessage so received
messages carry a stable cross-transport identity: (sender_pubkey,
sender_seq) pair. This is the foundation for the upcoming reply,
reaction, edit, and read-receipt variants — they need to target a
message by an ID that is meaningful on every node, not just locally.

Receive-side population lives in dispatch.rs::store_typed_message,
which now looks up the peer's pubkey_hex and copies envelope.seq from
the decoded TypedEnvelope. Sent-side population will land when we
plumb a per-node monotonic seq counter through the RPC layer.

Also adds mesh.debug-dump: a full in-memory state snapshot returning
peers, messages, status, shared-secret peer ids, encrypt_relay flag,
and stego mode — intended for smoke tests and bug investigation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

core/archipelago/src/api/rpc/dispatcher.rs

@@ -287,6 +287,7 @@ impl RpcHandler {
             "mesh.status" => self.handle_mesh_status().await,
             "mesh.peers" => self.handle_mesh_peers().await,
             "mesh.messages" => self.handle_mesh_messages(params).await,
+            "mesh.debug-dump" => self.handle_mesh_debug_dump().await,
             "mesh.send" => self.handle_mesh_send(params).await,
             "mesh.send-channel" => self.handle_mesh_send_channel(params).await,
             "mesh.broadcast" => self.handle_mesh_broadcast().await,