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_tone_test.py

@@ -0,0 +1,68 @@
+"""Test SoundBuffer tone generation."""
+import mcrfpy
+import sys
+
+# Test 1: Basic sine tone
+buf = mcrfpy.SoundBuffer.tone(440, 0.5, "sine")
+assert buf is not None
+assert abs(buf.duration - 0.5) < 0.02, f"Expected ~0.5s, got {buf.duration}"
+assert buf.sample_rate == 44100
+assert buf.channels == 1
+print("PASS: Sine tone 440Hz 0.5s")
+
+# Test 2: Square wave
+buf = mcrfpy.SoundBuffer.tone(220, 0.3, "square")
+assert abs(buf.duration - 0.3) < 0.02
+print("PASS: Square wave")
+
+# Test 3: Saw wave
+buf = mcrfpy.SoundBuffer.tone(330, 0.2, "saw")
+assert abs(buf.duration - 0.2) < 0.02
+print("PASS: Saw wave")
+
+# Test 4: Triangle wave
+buf = mcrfpy.SoundBuffer.tone(550, 0.4, "triangle")
+assert abs(buf.duration - 0.4) < 0.02
+print("PASS: Triangle wave")
+
+# Test 5: Noise
+buf = mcrfpy.SoundBuffer.tone(1000, 0.1, "noise")
+assert abs(buf.duration - 0.1) < 0.02
+print("PASS: Noise")
+
+# Test 6: ADSR envelope
+buf = mcrfpy.SoundBuffer.tone(440, 1.0, "sine",
+    attack=0.1, decay=0.2, sustain=0.5, release=0.3)
+assert abs(buf.duration - 1.0) < 0.02
+print("PASS: ADSR envelope")
+
+# Test 7: Custom sample rate
+buf = mcrfpy.SoundBuffer.tone(440, 0.5, "sine", sample_rate=22050)
+assert buf.sample_rate == 22050
+assert abs(buf.duration - 0.5) < 0.02
+print("PASS: Custom sample rate")
+
+# Test 8: Invalid waveform raises ValueError
+try:
+    mcrfpy.SoundBuffer.tone(440, 0.5, "invalid_waveform")
+    assert False, "Should have raised ValueError"
+except ValueError:
+    pass
+print("PASS: Invalid waveform raises ValueError")
+
+# Test 9: Negative duration raises ValueError
+try:
+    mcrfpy.SoundBuffer.tone(440, -0.5, "sine")
+    assert False, "Should have raised ValueError"
+except ValueError:
+    pass
+print("PASS: Negative duration raises ValueError")
+
+# Test 10: Samples are non-zero (tone actually generates audio)
+buf = mcrfpy.SoundBuffer.tone(440, 0.1, "sine")
+# In headless mode, sample_count should be nonzero
+assert buf.sample_count > 0, "Expected non-zero sample count"
+print(f"PASS: Tone has {buf.sample_count} samples")
+
+print("\nAll soundbuffer_tone tests passed!")
+sys.exit(0)