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>

tests/unit/soundbuffer_sound_test.py

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+"""Test Sound integration with SoundBuffer."""
+import mcrfpy
+import sys
+
+# Test 1: Sound accepts SoundBuffer
+buf = mcrfpy.SoundBuffer.tone(440, 0.5, "sine")
+sound = mcrfpy.Sound(buf)
+assert sound is not None
+print("PASS: Sound(SoundBuffer) works")
+
+# Test 2: Sound.buffer returns the SoundBuffer
+got_buf = sound.buffer
+assert got_buf is not None
+assert abs(got_buf.duration - buf.duration) < 0.02
+print("PASS: sound.buffer returns SoundBuffer")
+
+# Test 3: Sound.pitch property
+assert sound.pitch == 1.0, f"Default pitch should be 1.0, got {sound.pitch}"
+sound.pitch = 1.5
+assert abs(sound.pitch - 1.5) < 0.001
+sound.pitch = 1.0
+print("PASS: sound.pitch get/set")
+
+# Test 4: Sound.play_varied (in headless mode, just verifies no crash)
+sound.play_varied(pitch_range=0.1, volume_range=3.0)
+print("PASS: sound.play_varied() works")
+
+# Test 5: Sound from SoundBuffer has duration
+assert sound.duration > 0
+print(f"PASS: Sound from SoundBuffer has duration {sound.duration:.3f}s")
+
+# Test 6: Sound from SoundBuffer has source '<SoundBuffer>'
+assert sound.source == "<SoundBuffer>"
+print("PASS: Sound.source is '<SoundBuffer>' for buffer-created sounds")
+
+# Test 7: Backward compatibility - Sound still accepts string
+# File may not exist, so we test that a string is accepted (not TypeError)
+# and that RuntimeError is raised for missing files
+sound2 = None
+try:
+    sound2 = mcrfpy.Sound("test.ogg")
+    print("PASS: Sound(str) backward compatible (file loaded)")
+except RuntimeError:
+    # File doesn't exist - that's fine, the important thing is it accepted a string
+    print("PASS: Sound(str) backward compatible (raises RuntimeError for missing file)")
+
+# Test 8: Sound from SoundBuffer - standard playback controls
+sound.volume = 75.0
+assert abs(sound.volume - 75.0) < 0.1
+sound.loop = True
+assert sound.loop == True
+sound.loop = False
+print("PASS: Standard playback controls work with SoundBuffer")
+
+# Test 9: sfxr buffer -> Sound pipeline
+sfx = mcrfpy.SoundBuffer.sfxr("coin", seed=42)
+coin_sound = mcrfpy.Sound(sfx)
+assert coin_sound is not None
+assert coin_sound.duration > 0
+print(f"PASS: sfxr -> Sound pipeline ({coin_sound.duration:.3f}s)")
+
+# Test 10: Effect chain -> Sound pipeline
+processed = mcrfpy.SoundBuffer.tone(440, 0.3, "saw").low_pass(2000).normalize()
+proc_sound = mcrfpy.Sound(processed)
+assert proc_sound is not None
+assert proc_sound.duration > 0
+print(f"PASS: Effects -> Sound pipeline ({proc_sound.duration:.3f}s)")
+
+# Test 11: Sound with invalid argument type
+try:
+    mcrfpy.Sound(42)
+    assert False, "Should have raised TypeError"
+except TypeError:
+    pass
+print("PASS: Sound(int) raises TypeError")
+
+# Test 12: Sound.buffer is None for file-loaded sounds
+if sound2 is not None:
+    assert sound2.buffer is None
+    print("PASS: Sound.buffer is None for file-loaded sounds")
+else:
+    print("PASS: Sound.buffer test skipped (file not available)")
+
+print("\nAll soundbuffer_sound tests passed!")
+sys.exit(0)