Fix latent client-side encode_binary bounds min+size/min+max bug (nonzero position)

Found during review of the persistence-plumbing branch: _build_metadata_section
wrote raw bounds.size into the wire's 2nd float triple, but the wire format
(and _parse_metadata_section) treats that triple as max_pos, computing
size = max - min. AABB(Vector3.ZERO, size) masks this since size == max when
position is 0 -- exactly what every existing test used. Any chunk at a real,
nonzero world position would silently corrupt on its own client-local
encode/decode round-trip. Same bug class as the backend's min+size/min+max
mismatch fixed in chunk_codec.py (eb39e43), just on the client side this
time. Added a regression case to smoke_test_chunk_codec_interop.gd using a
real nonzero chunk position (96,-32,0) -- 33/33 passing. Full sibling smoke
suite re-verified clean afterward.

scripts/chunk_data_decoder.gd

@@ -492,12 +492,22 @@ static func _build_metadata_section(payload: ChunkPayload) -> PackedByteArray:
 	data.append_array(_write_int32(payload.coord.x))
 	data.append_array(_write_int32(payload.coord.y))
 	data.append_array(_write_int32(payload.coord.z))
+	# Wire format is min + max (world.md §3, and _parse_metadata_section
+	# below reads the 2nd triple as max_pos and computes size = max - min)
+	# -- writing raw `.size` here instead of `.position + .size` was a
+	# latent bug that only escaped every test because AABB(Vector3.ZERO, ...)
+	# happens to make size == max when position is exactly zero. Any chunk
+	# at a real, nonzero world position (i.e. every real chunk except the
+	# one at the origin) would round-trip through encode_binary/decode_binary
+	# with a corrupted size. Same bug class as the backend's min+size/
+	# min+max mismatch fixed in chunk_codec.py (commit eb39e43).
+	var bounds_max: Vector3 = payload.bounds.position + payload.bounds.size
 	data.append_array(_write_float32(payload.bounds.position.x))
 	data.append_array(_write_float32(payload.bounds.position.y))
 	data.append_array(_write_float32(payload.bounds.position.z))
-	data.append_array(_write_float32(payload.bounds.size.x))
+	data.append_array(_write_float32(bounds_max.x))
-	data.append_array(_write_float32(payload.bounds.size.y))
+	data.append_array(_write_float32(bounds_max.y))
-	data.append_array(_write_float32(payload.bounds.size.z))
+	data.append_array(_write_float32(bounds_max.z))
 	# Every other section builder wraps its body with `_wrap_section`
 	# (section_id + uint32 length prefix) -- this one didn't, so
 	# `encode_binary()` spliced the raw metadata bytes directly into the

scripts/smoke_test_chunk_codec_interop.gd

@@ -28,11 +28,36 @@ func _init() -> void:
 	_test_empty_chunk_fixture()
 	_test_simple_tile_chunk_fixture()
 	_test_chunk_with_objects_fixture()
+	_test_nonzero_bounds_roundtrip()
 
 	print("\n=== SMOKE RESULT: %d/%d passed ===" % [passed, passed + failed])
 	quit(0 if failed == 0 else 1)
 
 
+func _test_nonzero_bounds_roundtrip() -> void:
+	print("--- client-local encode_binary/decode_binary round-trip, nonzero bounds position ---")
+	# Regression for a real bug: _build_metadata_section used to write raw
+	# `bounds.size` into the wire's 2nd float triple, but the wire format
+	# (and _parse_metadata_section) treats that triple as max_pos, deriving
+	# size = max - min. AABB(Vector3.ZERO, size) masks this (size == max
+	# when position is 0), so this specifically uses a real, nonzero chunk
+	# position -- e.g. the chunk at coord (3,-1,0), matching the backend
+	# golden fixture's own bounds.position (96,-32,0) -- to catch it for real.
+	var payload := ChunkDataDecoder.ChunkPayload.new()
+	payload.world_id = "1"
+	payload.room_id = "main"
+	payload.coord = Vector3i(3, -1, 0)
+	payload.bounds = AABB(Vector3(96.0, -32.0, 0.0), Vector3(32.0, 32.0, 1.0))
+
+	var encoded := ChunkDataDecoder.encode_binary(payload)
+	var decoded: ChunkDataDecoder.ChunkPayload = ChunkDataDecoder.decode_binary(encoded)
+	_check("decoded without error", decoded != null)
+	if decoded == null:
+		return
+	_check("bounds.position survives round-trip (96,-32,0)", decoded.bounds.position.is_equal_approx(Vector3(96.0, -32.0, 0.0)))
+	_check("bounds.size survives round-trip (32,32,1) -- the actual regression check", decoded.bounds.size.is_equal_approx(Vector3(32.0, 32.0, 1.0)))
+
+
 func _read_fixture(name: String) -> PackedByteArray:
 	var f := FileAccess.open(FIXTURES_DIR + name, FileAccess.READ)
 	if f == null: