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/DynamicSoundFXPlayer2d.gd

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+@tool
+@icon("res://addons/gd-dynamic-sound/icon.svg")
+class_name DynamicSoundFXPlayer2D
+## 2D positional variant of [DynamicSoundFXPlayer].
+##
+## Each [method play_fx] call triggers a fresh play of the assigned
+## [DynamicSound], layered with any plays still in flight, distance-attenuated
+## from a 2D position relative to the listener.
+extends AudioStreamPlayer2D
+
+## The [DynamicSound] to trigger. Each [method play_fx] call plays a fresh instance.
+@export var Sound : DynamicSound;
+
+## Volume mix between intensity layers. Only blended when all three layers are set.
+@export_range(0, 1) var Intensity : float = 0;
+
+var _core : DynamicSoundFXPlayerCore;
+
+func _enter_tree() -> void:
+	if Engine.is_editor_hint() and stream == null:
+		stream = AudioStreamPolyphonic.new();
+
+func _ready() -> void:
+	_core = DynamicSoundFXPlayerCore.new(self);
+	_core.ready();
+
+## Trigger a one-shot of [member Sound]. Layers with any in-flight triggers.
+func play_fx() -> void:
+	if _core: _core.play_fx();