Initial commit: dynamic music & FX players

Continuous music players (DynamicSoundPlayer, 2D, 3D) with intensity-blended
layers, playlist queue, Loop, fade in/out, pause/resume. One-shot SFX players
(DynamicSoundFXPlayer, 2D, 3D) for overlapping triggers. All variants share
RefCounted core classes via duck-typed property access.

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

src/DynamicSoundPlaylist.gd

@@ -0,0 +1,8 @@
+@icon("res://addons/gd-dynamic-audio/src/dynamic_sound_playlist_icon.png")
+@tool
+class_name DynamicSoundPlaylist
+## The DynamicSound Holds onto all your music variants.
+extends Resource
+
+@export_category("Playlist")
+@export var QueuedSongs : Array[DynamicSound];