Fix 3 real client/server chunk-codec interop bugs, caught by a new golden-fixture test

No test ever exercised the client ChunkDataDecoder against real byte data
before this (M2 chunk streaming was never fed real content on PC2/PC3).
Added scripts/smoke_test_chunk_codec_interop.gd, decoding the backend's
exact golden fixtures (backend/tests/fixtures/chunks/*.bin) and asserting
byte-identical logical values -- the real cross-language contract this
whole persistence system depends on. It found three bugs on first run:

1. chunk_data_decoder.gd's _read_float32/_write_float32 used `bits as
   float`/`val as int`, a numeric cast in GDScript, not an IEEE-754
   bitcast -- silently corrupted every non-integer position/rotation/
   scale/bounds value (1.5 decoded as 1069547520.0, verified). Fixed with
   PackedByteArray.decode_float/encode_float (real bitcast, same
   little-endian order as the rest of this format).

2. backend/app/services/chunk_codec.py's binary codec wrote bounds as
   min+size, disagreeing with world.md §3's documented min+max wire
   format and with the client's actual (correct) decoder -- corrupted
   every chunk's bounds on the wire despite the backend's own JSON path
   using min+max correctly. Fixed the binary encode/decode to match the
   spec; regenerated the golden fixtures.

3. chunk_data_decoder.gd's object-properties parsing used
   `JSON.parse_string(props_str) or {}` -- GDScript's `or` is a boolean
   operator, not Python/JS-style short-circuit-return, so this silently
   evaluated to a bool for any object with real property data, crashing
   on assignment to the typed `properties: Dictionary` field. Fixed with
   an explicit type check.

Full backend suite (132/132) and 4 sibling client smoke tests re-verified
clean after all three fixes; no other `or {}`/`or []` instances of bug 3's
pattern found elsewhere in the codebase.

scripts/smoke_test_chunk_codec_interop.gd

@@ -0,0 +1,107 @@
+extends SceneTree
+## Proves the client-side ChunkDataDecoder decodes the backend's exact golden
+## fixture files (backend/tests/fixtures/chunks/*.bin) with byte-identical
+## logical values -- the actual cross-language contract this whole
+## persistence system depends on. Also regression-covers the float32
+## int<->float numeric-cast bug (fixed alongside this test): `bits as float`
+## silently corrupted every non-integer value (1.5 decoded as 1069547520.0).
+##
+## Run: ~/godot/godot --headless --script res://scripts/smoke_test_chunk_codec_interop.gd
+
+const FIXTURES_DIR := "res://backend/tests/fixtures/chunks/"
+
+var passed := 0
+var failed := 0
+
+
+func _check(label: String, cond: bool) -> void:
+	if cond:
+		passed += 1
+		print("  PASS: ", label)
+	else:
+		failed += 1
+		print("  FAIL: ", label)
+
+
+func _init() -> void:
+	_test_float32_roundtrip()
+	_test_empty_chunk_fixture()
+	_test_simple_tile_chunk_fixture()
+	_test_chunk_with_objects_fixture()
+
+	print("\n=== SMOKE RESULT: %d/%d passed ===" % [passed, passed + failed])
+	quit(0 if failed == 0 else 1)
+
+
+func _read_fixture(name: String) -> PackedByteArray:
+	var f := FileAccess.open(FIXTURES_DIR + name, FileAccess.READ)
+	if f == null:
+		push_error("Could not open fixture: " + name)
+		return PackedByteArray()
+	return f.get_buffer(f.get_length())
+
+
+func _test_float32_roundtrip() -> void:
+	print("--- float32 bitcast regression (the actual bug this fix addresses) ---")
+	for val in [1.5, -110.25, 0.5, 3.14159, -0.001, 0.0, 100.0]:
+		var encoded: PackedByteArray = ChunkDataDecoder._write_float32(val)
+		var decoded: float = ChunkDataDecoder._read_float32(encoded, 0)
+		_check("float32 round-trip for %s (got %s)" % [val, decoded], is_equal_approx(decoded, val))
+	# The specific regression case: 1.5 must NOT decode as 1069547520.0
+	# (the old `bits as float` numeric-cast bug's actual observed output).
+	var bad_encoded := ChunkDataDecoder._write_float32(1.5)
+	var bad_decoded: float = ChunkDataDecoder._read_float32(bad_encoded, 0)
+	_check("1.5 does not decode as the old numeric-cast bug's 1069547520.0", not is_equal_approx(bad_decoded, 1069547520.0))
+
+
+func _test_empty_chunk_fixture() -> void:
+	print("--- backend golden fixture: empty_chunk.bin ---")
+	var data := _read_fixture("empty_chunk.bin")
+	_check("fixture file read (non-empty)", data.size() > 0)
+	var payload: ChunkDataDecoder.ChunkPayload = ChunkDataDecoder.decode_binary(data)
+	_check("decoded without error", payload != null)
+	if payload == null:
+		return
+	_check("world_id matches backend fixture (7)", payload.world_id == "7")
+	_check("no tile layers", payload.tile_layers.is_empty())
+	_check("no objects", payload.objects.is_empty())
+	_check("revision matches backend fixture (1)", payload.revision == 1)
+
+
+func _test_simple_tile_chunk_fixture() -> void:
+	print("--- backend golden fixture: simple_tile_chunk.bin ---")
+	var data := _read_fixture("simple_tile_chunk.bin")
+	var payload: ChunkDataDecoder.ChunkPayload = ChunkDataDecoder.decode_binary(data)
+	_check("decoded without error", payload != null)
+	if payload == null:
+		return
+	_check("world_id matches (12)", payload.world_id == "12")
+	_check("coord matches (3,-1,0)", payload.coord == Vector3i(3, -1, 0))
+	_check("bounds.position matches (96.0,-32.0,0.0) -- non-trivial floats", payload.bounds.position.is_equal_approx(Vector3(96.0, -32.0, 0.0)))
+	_check("bounds.size matches (32.0,32.0,1.0)", payload.bounds.size.is_equal_approx(Vector3(32.0, 32.0, 1.0)))
+	_check("one tile layer parsed", payload.tile_layers.size() == 1)
+	if payload.tile_layers.size() == 1:
+		var layer: ChunkDataDecoder.TileLayerData = payload.tile_layers[0]
+		_check("tile layer dims 4x4x1", layer.width == 4 and layer.height == 4 and layer.depth == 1)
+		_check("tile data matches backend fixture", layer.tiles == PackedInt32Array([0, 1, 1, 0, 1, 2, 2, 1, 1, 2, 2, 1, 0, 1, 1, 0]))
+	_check("revision matches (2)", payload.revision == 2)
+
+
+func _test_chunk_with_objects_fixture() -> void:
+	print("--- backend golden fixture: chunk_with_objects.bin (real non-integer floats) ---")
+	var data := _read_fixture("chunk_with_objects.bin")
+	var payload: ChunkDataDecoder.ChunkPayload = ChunkDataDecoder.decode_binary(data)
+	_check("decoded without error", payload != null)
+	if payload == null:
+		return
+	_check("one object parsed", payload.objects.size() >= 1)
+	if payload.objects.is_empty():
+		return
+	var obj: ChunkDataDecoder.WorldObjectData = payload.objects[0]
+	_check("object id matches (obj_0001)", obj.id == "obj_0001")
+	_check("object asset_id matches (props/chest)", obj.asset_id == "props/chest")
+	# The core regression check: these are exactly the kind of non-integer
+	# values that decoded as garbage under the old `bits as float` bug.
+	_check("object position.x matches 101.5 exactly (was the corrupted value)", is_equal_approx(obj.position.x, 101.5))
+	_check("object position.y matches -110.25 exactly (was the corrupted value)", is_equal_approx(obj.position.y, -110.25))
+	_check("object position.z matches 0.5 exactly (was the corrupted value)", is_equal_approx(obj.position.z, 0.5))