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/protocol/protocol.gd

@@ -15,17 +15,31 @@ static func _mp():
 # -- Decode: bytes from server → GDScript types --------------------------------
 
 
+## Decode a raw MessagePack frame to its top-level value (or null on error).
+## #960: lets receive_bytes decode a frame once and branch by shape before
+## committing to the heavier snapshot decode.
+static func decode_raw(bytes: PackedByteArray) -> Variant:
+	var result = _mp().decode(bytes)
+	if result.status != null:
+		push_error("Protocol: msgpack decode failed: %s" % result.status)
+		return null
+	return result.value
+
+
 ## Decode an ObserverSnapshot from MessagePack bytes.
 ## Returns decoded snapshot Dictionary or null on error.
 ## v2 fields (version, game_time, player_facing, visible_tiles) default to null/empty
 ## when decoding v1 snapshots for backward compatibility.
 static func decode_snapshot(bytes: PackedByteArray) -> Variant:
-	var result = _mp().decode(bytes)
-	if result.status != null:
-		push_error("Protocol: msgpack decode failed: %s" % result.status)
+	var raw = decode_raw(bytes)
+	if raw == null:
 		return null
+	return _decode_snapshot_from_raw(raw)
 
-	var raw = result.value
+
+## Build an ObserverSnapshot from an already-decoded raw value (the snapshot
+## body, shared by decode_snapshot and the receive-side classifier).
+static func _decode_snapshot_from_raw(raw: Variant) -> Variant:
 	if not raw is Dictionary or not raw.has("tick") or not raw.has("entities"):
 		push_error("Protocol: snapshot missing required fields")
 		return null
@@ -745,10 +759,12 @@ static func encode_atlas_layer_request(body_id: String, up_to: String = "Topogra
 ## name ("Ready"|"Pending"|"NotFound"|"Error"); error holds the message for the
 ## Error variant. layer1 is the raw decoded Layer1Output map, or null.
 static func decode_atlas_layer_response(bytes: PackedByteArray) -> Variant:
-	var result = _mp().decode(bytes)
-	if result.status != null:
-		return null
-	var raw = result.value
+	return atlas_response_from_raw(decode_raw(bytes))
+
+
+## Build an AtlasLayerResponse from an already-decoded raw value. Returns null
+## if it is not an atlas response (no "status" key).
+static func atlas_response_from_raw(raw: Variant) -> Variant:
 	if not raw is Dictionary or not raw.has("status"):
 		return null
 	var status_raw = raw["status"]
@@ -767,6 +783,20 @@ static func decode_atlas_layer_response(bytes: PackedByteArray) -> Variant:
 	}
 
 
+## Decode + classify one inbound frame (#960, D-225). Returns {kind, value} with
+## kind "snapshot" | "atlas" | "unknown" — both are msgpack maps, so they are
+## told apart by field (a response has "status"; a snapshot has "entities").
+## Lets receive_bytes decode the frame ONCE and branch, instead of double-decoding
+## the 20 Hz snapshot path.
+static func decode_inbound(bytes: PackedByteArray) -> Dictionary:
+	var raw = decode_raw(bytes)
+	if not raw is Dictionary:
+		return {"kind": "unknown", "value": null}
+	if raw.has("status") and not raw.has("entities"):
+		return {"kind": "atlas", "value": atlas_response_from_raw(raw)}
+	return {"kind": "snapshot", "value": _decode_snapshot_from_raw(raw)}
+
+
 ## Encode a ConfirmBookmark action (#614, #680).
 ## Struct variant with bookmark_id and starting_location_id.
 static func encode_confirm_bookmark(bookmark_id: String, starting_location_id: String) -> PackedByteArray: