auto-detect Music/FX bus in editor

When a player enters the tree in the editor, if its bus is still the default
"Master" and a "Music" (or "FX") bus exists in the AudioServer, switch to that.
User-customised buses are not overridden.

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

src/DynamicSoundFXPlayer2d.gd

@@ -17,8 +17,12 @@ extends AudioStreamPlayer2D
 var _core : DynamicSoundFXPlayerCore;
 
 func _enter_tree() -> void:
-	if Engine.is_editor_hint() and stream == null:
+	if not Engine.is_editor_hint():
+		return;
+	if stream == null:
 		stream = AudioStreamPolyphonic.new();
+	if bus == &"Master" and AudioServer.get_bus_index("FX") != -1:
+		bus = "FX";
 
 func _ready() -> void:
 	_core = DynamicSoundFXPlayerCore.new(self);