respect autoplay; add play_playlist() for manual start

DynamicSoundPlayerCore.ready() always called play() if a playlist had songs,
ignoring the inherited AudioStreamPlayer.autoplay property. Now early-returns
when autoplay is false. Adds play_playlist() (exposed on all three continuous
wrappers) so users with autoplay=false have a non-fade way to start playback.
fade_in() now uses the same _start_playback() helper.

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

src/DynamicSoundPlayer2d.gd

@@ -44,6 +44,10 @@ func _ready() -> void:
 func _process(delta: float) -> void:
 	if _core: _core.process(delta);
 
+## Start (or resume) playlist playback. Useful when [code]autoplay[/code] is false.
+func play_playlist() -> void:
+	if _core: _core.play_playlist();
+
 ## Pause playback. Call [method resume] to continue.
 func pause() -> void:
 	if _core: _core.pause();