feat(client): atlas layer-stream codec — encode request / decode response (#960, D-225)

Client half of the layer-stream protocol (codec only; transport wiring next).
- protocol.gd: encode_atlas_layer_request (bare {body_id, up_to} map so the
  server demux routes it to the proxy, not the PlayerInput array) and
  decode_atlas_layer_response (-> {body_id, status, error, layer1}; returns null
  for non-atlas frames, e.g. a snapshot, so receive_bytes can disambiguate).
- gen_fixtures.rs: cross-language fixtures (atlas_response_ready/pending/
  not_found) from real rmp_serde output, matching the test_protocol.gd pattern.
- test_protocol.gd: 5 tests decode the fixtures + verify a snapshot is not
  mistaken for a response + the request encodes to the right shape. 68/68 pass.

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

client/tests/fixtures/msgpack/atlas_response_not_found.msgpack

@@ -0,0 +1 @@
+ƒ§body_id¥ghost¦status¨NotFound¦layer1À

client/tests/fixtures/msgpack/atlas_response_pending.msgpack

@@ -0,0 +1 @@
+�body_id二J1c存tatus判ending奸ayer1�

client/tests/test_protocol.gd

@@ -463,3 +463,51 @@ func test_encode_confirm_bookmark_roundtrip() -> void:
 	var data: Dictionary = entry["action_data"]
 	assert_that(data["bookmark_id"]).is_equal("bm_tycoon_arion")
 	assert_that(data["starting_location_id"]).is_equal("loc_arion_prime")
+
+
+# -- Atlas layer-stream protocol (#969, D-225) ---------------------------------
+
+func test_decode_atlas_response_ready() -> void:
+	var bytes := _load_fixture("atlas_response_ready")
+	var resp = Protocol.decode_atlas_layer_response(bytes)
+	assert_that(resp).is_not_null()
+	assert_that(resp.body_id).is_equal("GJ1c")
+	assert_that(resp.status).is_equal("Ready")
+	assert_that(resp.layer1).is_not_null()
+	assert_that(resp.layer1.river_network.river_cells.size()).is_equal(2)
+	assert_that(resp.layer1.attractors.size()).is_equal(1)
+	assert_that(resp.layer1.attractors[0].attractor_type).is_equal("CoastalAccess")
+	assert_that(resp.layer1.attractors[0].sub_biome).is_equal("CoastalLowland")
+
+
+func test_decode_atlas_response_pending() -> void:
+	var bytes := _load_fixture("atlas_response_pending")
+	var resp = Protocol.decode_atlas_layer_response(bytes)
+	assert_that(resp).is_not_null()
+	assert_that(resp.status).is_equal("Pending")
+	assert_that(resp.layer1).is_null()
+
+
+func test_decode_atlas_response_not_found() -> void:
+	var bytes := _load_fixture("atlas_response_not_found")
+	var resp = Protocol.decode_atlas_layer_response(bytes)
+	assert_that(resp.status).is_equal("NotFound")
+
+
+func test_snapshot_is_not_decoded_as_atlas_response() -> void:
+	# Disambiguation: an ObserverSnapshot has no "status" key, so the atlas
+	# decoder rejects it. receive_bytes relies on this to route correctly.
+	var bytes := _load_fixture("snapshot_empty")
+	assert_that(Protocol.decode_atlas_layer_response(bytes)).is_null()
+
+
+func test_encode_atlas_request_shape() -> void:
+	var bytes := Protocol.encode_atlas_layer_request("GJ1c", "Topography")
+	assert_that(bytes.size()).is_greater(0)
+	var raw: Variant = Messagepack.decode(bytes)
+	assert_that(raw.status).is_null()
+	assert_that(raw.value is Dictionary).is_true()
+	assert_that(raw.value["body_id"]).is_equal("GJ1c")
+	assert_that(raw.value["up_to"]).is_equal("Topography")
+	# A request is a map with no "status" — must not be mistaken for a response.
+	assert_that(Protocol.decode_atlas_layer_response(bytes)).is_null()