move PitchMode to FX players and implement random pitch

The continuous music players had PitchMode/RandomMin/MaxPitchScale exports but
no logic referenced them. Pitch variation only makes sense per-trigger, which
the FX family supports natively. So:

- Remove PitchMode, RandomMinPitchScale, RandomMaxPitchScale, and the
  _validate_property hook from DynamicSoundPlayer / 2D / 3D.
- Add the same exports + _validate_property to DynamicSoundFXPlayer / 2D / 3D.
- Implement pitch resolution in DynamicSoundFXPlayerCore.play_fx: each trigger
  samples a fresh pitch from PitchMode and passes it as the pitch_scale
  argument to AudioStreamPlaybackPolyphonic.play_stream.
- Random pitch uses 1.0 + randf_range(min, max) so the existing -0.5/0.5
  defaults give a useful [0.5, 1.5] pitch range.
- README updated to reflect the move.

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

src/DynamicSoundFXPlayer2d.gd

@@ -14,6 +14,17 @@ extends AudioStreamPlayer2D
 ## Volume mix between intensity layers. Only blended when all three layers are set.
 @export_range(0, 1) var Intensity : float = 0;
 
+## How playback pitch is selected for each [method play_fx] trigger.
+@export var PitchMode : DynamicSoundConstants.PitchMode = DynamicSoundConstants.PitchMode.Constant:
+	set(value):
+		PitchMode = value;
+		notify_property_list_changed();
+
+## Minimum pitch offset (added to 1.0) when [member PitchMode] is Random.
+@export var RandomMinPitchScale : float = -0.5;
+## Maximum pitch offset (added to 1.0) when [member PitchMode] is Random.
+@export var RandomMaxPitchScale : float = 0.5;
+
 var _core : DynamicSoundFXPlayerCore;
 
 func _enter_tree() -> void:
@@ -24,6 +35,11 @@ func _enter_tree() -> void:
 	if bus == &"Master" and AudioServer.get_bus_index("FX") != -1:
 		bus = "FX";
 
+func _validate_property(property: Dictionary) -> void:
+	if property.name in ["RandomMinPitchScale", "RandomMaxPitchScale"]:
+		if PitchMode != DynamicSoundConstants.PitchMode.Random:
+			property.usage &= ~PROPERTY_USAGE_EDITOR;
+
 func _ready() -> void:
 	_core = DynamicSoundFXPlayerCore.new(self);
 	_core.ready();