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_sfxr_test.py

@@ -0,0 +1,94 @@
+"""Test SoundBuffer sfxr synthesis."""
+import mcrfpy
+import sys
+
+# Test 1: All presets work
+presets = ["coin", "laser", "explosion", "powerup", "hurt", "jump", "blip"]
+for preset in presets:
+    buf = mcrfpy.SoundBuffer.sfxr(preset)
+    assert buf is not None, f"sfxr('{preset}') returned None"
+    assert buf.sample_count > 0, f"sfxr('{preset}') has 0 samples"
+    assert buf.duration > 0, f"sfxr('{preset}') has 0 duration"
+    assert buf.sfxr_params is not None, f"sfxr('{preset}') has no params"
+    print(f"  PASS: sfxr('{preset}') -> {buf.duration:.3f}s, {buf.sample_count} samples")
+
+print("PASS: All sfxr presets generate audio")
+
+# Test 2: Deterministic with seed
+buf1 = mcrfpy.SoundBuffer.sfxr("coin", seed=42)
+buf2 = mcrfpy.SoundBuffer.sfxr("coin", seed=42)
+assert buf1.sample_count == buf2.sample_count, "Same seed should produce same sample count"
+assert buf1.duration == buf2.duration, "Same seed should produce same duration"
+print("PASS: Deterministic with seed")
+
+# Test 3: Different seeds produce different results
+buf3 = mcrfpy.SoundBuffer.sfxr("coin", seed=99)
+# May have same count by chance, but params should differ
+p1 = buf1.sfxr_params
+p3 = buf3.sfxr_params
+# At least one param should differ (with very high probability)
+differs = any(p1[k] != p3[k] for k in p1.keys() if k != 'wave_type')
+assert differs, "Different seeds should produce different params"
+print("PASS: Different seeds produce different results")
+
+# Test 4: sfxr_params dict contains expected keys
+params = buf1.sfxr_params
+expected_keys = [
+    'wave_type', 'base_freq', 'freq_limit', 'freq_ramp', 'freq_dramp',
+    'duty', 'duty_ramp', 'vib_strength', 'vib_speed',
+    'env_attack', 'env_sustain', 'env_decay', 'env_punch',
+    'lpf_freq', 'lpf_ramp', 'lpf_resonance',
+    'hpf_freq', 'hpf_ramp',
+    'pha_offset', 'pha_ramp', 'repeat_speed',
+    'arp_speed', 'arp_mod'
+]
+for key in expected_keys:
+    assert key in params, f"Missing key '{key}' in sfxr_params"
+print("PASS: sfxr_params has all expected keys")
+
+# Test 5: sfxr with custom params
+buf_custom = mcrfpy.SoundBuffer.sfxr(wave_type=2, base_freq=0.5, env_decay=0.3)
+assert buf_custom is not None
+assert buf_custom.sfxr_params is not None
+assert buf_custom.sfxr_params['wave_type'] == 2
+assert abs(buf_custom.sfxr_params['base_freq'] - 0.5) < 0.001
+print("PASS: sfxr with custom params")
+
+# Test 6: sfxr_mutate
+mutated = buf1.sfxr_mutate(0.1)
+assert mutated is not None
+assert mutated.sfxr_params is not None
+assert mutated.sample_count > 0
+# Params should be similar but different
+mp = mutated.sfxr_params
+op = buf1.sfxr_params
+differs = any(abs(mp[k] - op[k]) > 0.0001 for k in mp.keys() if isinstance(mp[k], float))
+# Note: with small mutation and few params, there's a chance all stay same.
+# But with 0.1 amount and ~20 float params, extremely unlikely all stay same.
+print(f"PASS: sfxr_mutate produces {'different' if differs else 'similar'} params")
+
+# Test 7: sfxr_mutate with seed for reproducibility
+m1 = buf1.sfxr_mutate(0.05, 42)
+m2 = buf1.sfxr_mutate(0.05, 42)
+assert m1.sample_count == m2.sample_count, "Same seed should produce same mutation"
+print("PASS: sfxr_mutate deterministic with seed")
+
+# Test 8: sfxr_mutate on non-sfxr buffer raises error
+tone_buf = mcrfpy.SoundBuffer.tone(440, 0.5, "sine")
+try:
+    tone_buf.sfxr_mutate(0.1)
+    assert False, "Should have raised RuntimeError"
+except RuntimeError:
+    pass
+print("PASS: sfxr_mutate on non-sfxr buffer raises RuntimeError")
+
+# Test 9: Invalid preset raises ValueError
+try:
+    mcrfpy.SoundBuffer.sfxr("nonexistent_preset")
+    assert False, "Should have raised ValueError"
+except ValueError:
+    pass
+print("PASS: Invalid preset raises ValueError")
+
+print("\nAll soundbuffer_sfxr tests passed!")
+sys.exit(0)