Add SoundBuffer type: procedural audio, sfxr synthesis, DSP effects

New SoundBuffer Python type enables procedural audio generation:
- Tone synthesis (sine, square, saw, triangle, noise) with ADSR envelopes
- sfxr retro sound effect engine (7 presets, 24 params, mutation, seeding)
- DSP effects chain: pitch_shift, low/high pass, echo, reverb,
  distortion, bit_crush, normalize, reverse, slice
- Composition: concat (with crossfade overlap) and mix
- Sound() now accepts SoundBuffer or filename string
- Sound gains pitch property and play_varied() method
- Platform stubs for HeadlessTypes and SDL2Types (loadFromSamples, pitch)
- Interactive demo: sfxr clone UI + Animalese speech synthesizer
- 62 unit tests across 6 test files (all passing)

Refs #251

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/McRFPy_API.cpp

@@ -17,6 +17,7 @@
 #include "PyMouseButton.h"
 #include "PyInputState.h"
 #include "PySound.h"
+#include "PySoundBuffer.h"
 #include "PyMusic.h"
 #include "PyKeyboard.h"
 #include "PyMouse.h"
@@ -467,6 +468,7 @@ PyObject* PyInit_mcrfpy()
 
         /*audio (#66)*/
         &PySoundType,
+        &PySoundBufferType,
         &PyMusicType,
 
         /*keyboard state (#160)*/