fix(world): correct 3D chunk coordinate mapping and streaming proximity

scripts/smoke_test_chunk_streaming_coords.gd

@@ -0,0 +1,109 @@
+extends SceneTree
+## Smoke test verifying chunk coordinate mapping and streaming behavior.
+## Regression test for the issue where moving forward in Z unloaded chunk (0,0,0)
+## because of transposed Godot Y (Height) and Godot Z (Ground) in coord_for_position.
+
+var passed := 0
+var failed := 0
+
+
+func _check(label: String, cond: bool) -> void:
+	if cond:
+		passed += 1
+		print("  PASS: ", label)
+	else:
+		failed += 1
+		printerr("  FAIL: ", label)
+
+
+func _init() -> void:
+	_test_coord_for_position()
+	_test_chunk_stream_manager_proximity()
+	print("\n=== SMOKE RESULT: %d/%d passed ===" % [passed, passed + failed])
+	quit(0 if failed == 0 else 1)
+
+
+func _test_coord_for_position() -> void:
+	print("--- Chunk.coord_for_position coordinate mapping ---")
+	# In Godot 3D: X is East/West (chunk X), Y is Height/Up (chunk Z), Z is North/South (chunk Y).
+
+	# Spawn origin
+	var c0 := Chunk.coord_for_position(Vector3(0.0, 1.5, 0.0))
+	_check("Origin at (0, 1.5, 0) maps to (0, 0, 0)", c0 == Vector3i(0, 0, 0))
+
+	# Moving along Godot Z (North/South) within chunk 0:
+	# Previously at Z=3, coord.z was incorrectly 3, which exceeded streaming_radius and unloaded chunk 0!
+	var c3 := Chunk.coord_for_position(Vector3(0.0, 1.5, 3.0))
+	_check("Walking 3m forward to Z=3 maps to chunk (0, 0, 0)", c3 == Vector3i(0, 0, 0))
+
+	var c20 := Chunk.coord_for_position(Vector3(10.0, 1.5, 20.0))
+	_check("Inside chunk at (10, 1.5, 20) maps to (0, 0, 0)", c20 == Vector3i(0, 0, 0))
+
+	var c31 := Chunk.coord_for_position(Vector3(31.0, 1.5, 31.0))
+	_check("Far corner of chunk at (31, 1.5, 31) maps to (0, 0, 0)", c31 == Vector3i(0, 0, 0))
+
+	# Crossing into adjacent chunks
+	var cz_next := Chunk.coord_for_position(Vector3(0.0, 1.5, 32.0))
+	_check("Crossing Z=32 maps to chunk Y=1 (0, 1, 0)", cz_next == Vector3i(0, 1, 0))
+
+	var cx_next := Chunk.coord_for_position(Vector3(32.0, 1.5, 0.0))
+	_check("Crossing X=32 maps to chunk X=1 (1, 0, 0)", cx_next == Vector3i(1, 0, 0))
+
+	var cz_prev := Chunk.coord_for_position(Vector3(0.0, 1.5, -1.0))
+	_check("Negative Z maps to chunk Y=-1 (0, -1, 0)", cz_prev == Vector3i(0, -1, 0))
+
+	var cx_prev := Chunk.coord_for_position(Vector3(-1.0, 1.5, 0.0))
+	_check("Negative X maps to chunk X=-1 (-1, 0, 0)", cx_prev == Vector3i(-1, 0, 0))
+
+	# Player height / jumping:
+	var c_jump := Chunk.coord_for_position(Vector3(0.0, 5.0, 0.0))
+	_check("Jumping to height Y=5 in flat world stays at cz=0", c_jump == Vector3i(0, 0, 0))
+
+
+func _test_chunk_stream_manager_proximity() -> void:
+	print("--- ChunkStreamManager streaming proximity ---")
+	var sm := ChunkStreamManager.new()
+	sm.streaming_radius = 2
+
+	var player := Node3D.new()
+	player.position = Vector3(0.0, 1.5, 0.0)
+	root.add_child(player)
+	root.add_child(sm)
+
+	sm.player = player
+	sm.is_active = true
+
+	# Simulate immediate streaming update on spawn
+	sm._update_streaming()
+	var targets: Dictionary = sm._compute_target_chunks()
+
+	_check("Target chunks computed on spawn", targets.size() > 0)
+	_check("Spawn chunk (0,0,0) is in target set", targets.has(Vector3i(0, 0, 0)))
+
+	# Ensure flat world doesn't spam non-zero cz chunks
+	var all_cz_zero := true
+	for coord in targets.keys():
+		if coord.z != 0:
+			all_cz_zero = false
+			break
+	_check("Flat world targets only contain cz=0", all_cz_zero)
+
+	# Simulate moving close to placed lampposts (Z = 1, 2, 3, 4, 5, 10, 20)
+	player.position = Vector3(0.0, 1.5, 3.0)
+	sm._update_streaming()
+	targets = sm._compute_target_chunks()
+	_check("Approaching lamppost at Z=3: chunk (0,0,0) is STILL in target set", targets.has(Vector3i(0, 0, 0)))
+
+	player.position = Vector3(5.0, 1.5, 15.0)
+	sm._update_streaming()
+	targets = sm._compute_target_chunks()
+	_check("Approaching lamppost at Z=15: chunk (0,0,0) is STILL in target set", targets.has(Vector3i(0, 0, 0)))
+
+	player.position = Vector3(0.0, 1.5, 31.0)
+	sm._update_streaming()
+	targets = sm._compute_target_chunks()
+	_check("Approaching lamppost at Z=31: chunk (0,0,0) is STILL in target set", targets.has(Vector3i(0, 0, 0)))
+
+	# Clean up
+	sm.queue_free()
+	player.queue_free()