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

scripts/chunk_stream_manager.gd

@@ -58,6 +58,7 @@ var player: Node3D = null
 
 ## The current player chunk coordinate.
 var _player_chunk_coord: Vector3i = Vector3i.ZERO
+var _has_player_coord: bool = false
 
 ## Map of loaded chunk coordinates to their scene nodes.
 var _loaded_chunks: Dictionary = {}
@@ -95,7 +96,7 @@ func _process(delta: float) -> void:
 		return
 
 	_update_accumulator += delta
-	if _update_accumulator < UPDATE_INTERVAL:
+	if _has_player_coord and _update_accumulator < UPDATE_INTERVAL:
 		return
 	_update_accumulator = 0.0
 
@@ -110,9 +111,10 @@ func _update_streaming() -> void:
 		return
 
 	var current_coord: Vector3i = Chunk.coord_for_position(player.global_position)
-	if current_coord == _player_chunk_coord:
+	if _has_player_coord and current_coord == _player_chunk_coord:
 		return
 
+	_has_player_coord = true
 	_player_chunk_coord = current_coord
 	_rebuild_queues()
 
@@ -144,9 +146,11 @@ func _compute_target_chunks() -> Dictionary:
 	"""Compute the set of chunk coordinates that should be loaded."""
 	var target: Dictionary = {}
 	var half: int = streaming_radius
+	var z_min: int = -half if Chunk.CHUNK_SIZE_Z > 1 else 0
+	var z_max: int = half if Chunk.CHUNK_SIZE_Z > 1 else 0
 	for dx in range(-half, half + 1):
 		for dy in range(-half, half + 1):
-			for dz in range(-half, half + 1):
+			for dz in range(z_min, z_max + 1):
 				var coord: Vector3i = Vector3i(
 					_player_chunk_coord.x + dx,
 					_player_chunk_coord.y + dy,