feat(client): atlas request/response transport on the bridge (#960, D-225)

Client transport half of the layer-stream protocol.
- SimBridge.request_atlas_layers(body_id) sends an AtlasLayerRequest frame
  (live mode only; no-op in test mode); responses arrive via a new
  atlas_layers_received signal.
- receive_bytes now decodes each frame ONCE via Protocol.decode_inbound and
  branches by shape (snapshot vs atlas response) — avoids double-decoding the
  20 Hz snapshot path. decode_snapshot is split into decode_raw +
  _decode_snapshot_from_raw (public decode_snapshot unchanged, so the 70 protocol
  tests stay the regression guard); decode_inbound returns {kind, value}.

70/70 protocol tests pass, including the new decode_inbound classifier test.
(Pre-existing client-suite failures in server-dependent e2e/roundtrip + unrelated
audio/fog/dialogue suites are unchanged — verified identical at baseline.)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

client/scripts/autoloads/sim_bridge.gd

@@ -3,6 +3,7 @@ extends Node
 # Signals
 signal connection_state_changed(old_state: ConnectionState, new_state: ConnectionState)
 signal snapshot_received(snapshot: Dictionary)
+signal atlas_layers_received(response: Dictionary)
 signal handshake_complete
 signal handshake_failed(reason: String)
 
@@ -387,6 +388,22 @@ func send_named_action(action_name: String, action_data: Variant = null) -> void
 	_outbound_buffer.append(entry)
 
 
+## Request a body's generation-cascade layers from the server (#960, D-225).
+## Live mode only — sends an AtlasLayerRequest frame; the response arrives via the
+## atlas_layers_received signal. No-op in test mode (no server connection).
+func request_atlas_layers(body_id: String, up_to: String = "Topography") -> void:
+	if test_mode or _bridge == null or state != ConnectionState.CONNECTED:
+		return
+	var bytes := Protocol.encode_atlas_layer_request(body_id, up_to)
+	if bytes.is_empty():
+		return
+	var err: int = _bridge.send_message(bytes)
+	if err != OK:
+		push_error(
+			"SimBridge: failed to send atlas layer request for %s: %s" % [body_id, error_string(err)]
+		)
+
+
 # Poll for snapshot from simulation.
 # In test mode delegates to test harness. In live mode, returns the last decoded snapshot.
 func poll_snapshot() -> Variant:
@@ -412,9 +429,18 @@ func poll_snapshot() -> Variant:
 # events (monologue, dialogue) are carried forward from overwritten snapshots
 # so they aren't silently dropped when server ticks faster than client consumes.
 func receive_bytes(bytes: PackedByteArray) -> void:
-	var snapshot = Protocol.decode_snapshot(bytes)
+	# Decode once, branch by frame shape (#960, D-225): atlas layer responses and
+	# snapshots are both msgpack maps, told apart by field.
+	var inbound := Protocol.decode_inbound(bytes)
+	if inbound.kind == "atlas":
+		atlas_layers_received.emit(inbound.value)
+		return
+	if inbound.kind != "snapshot":
+		push_warning("SimBridge: undecodable frame (%d bytes)" % bytes.size())
+		return
+	var snapshot = inbound.value
 	if snapshot == null:
-		push_warning("SimBridge: decode_snapshot returned null for %d bytes" % bytes.size())
+		push_warning("SimBridge: snapshot decode returned null for %d bytes" % bytes.size())
 		return
 	if _last_snapshot != null:
 		# Carry forward one-shot events the client hasn't consumed yet.