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_effects_test.py

@@ -0,0 +1,102 @@
+"""Test SoundBuffer DSP effects."""
+import mcrfpy
+import sys
+
+# Create a test buffer: 0.5s 440Hz sine
+src = mcrfpy.SoundBuffer.tone(440, 0.5, "sine")
+
+# Test 1: pitch_shift
+higher = src.pitch_shift(2.0)
+assert higher is not None
+assert higher.sample_count > 0
+# Higher pitch = shorter duration
+assert higher.duration < src.duration, f"pitch_shift(2.0) should be shorter: {higher.duration} vs {src.duration}"
+print(f"PASS: pitch_shift(2.0) -> {higher.duration:.3f}s (was {src.duration:.3f}s)")
+
+lower = src.pitch_shift(0.5)
+assert lower.duration > src.duration, f"pitch_shift(0.5) should be longer: {lower.duration} vs {src.duration}"
+print(f"PASS: pitch_shift(0.5) -> {lower.duration:.3f}s")
+
+# Test 2: low_pass
+lp = src.low_pass(500.0)
+assert lp is not None
+assert lp.sample_count == src.sample_count
+assert lp.duration == src.duration
+print("PASS: low_pass preserves sample count and duration")
+
+# Test 3: high_pass
+hp = src.high_pass(500.0)
+assert hp is not None
+assert hp.sample_count == src.sample_count
+print("PASS: high_pass preserves sample count")
+
+# Test 4: echo
+echoed = src.echo(200.0, 0.4, 0.5)
+assert echoed is not None
+assert echoed.sample_count == src.sample_count  # same length
+print("PASS: echo works")
+
+# Test 5: reverb
+reverbed = src.reverb(0.8, 0.5, 0.3)
+assert reverbed is not None
+assert reverbed.sample_count == src.sample_count
+print("PASS: reverb works")
+
+# Test 6: distortion
+dist = src.distortion(2.0)
+assert dist is not None
+assert dist.sample_count == src.sample_count
+print("PASS: distortion works")
+
+# Test 7: bit_crush
+crushed = src.bit_crush(8, 4)
+assert crushed is not None
+assert crushed.sample_count == src.sample_count
+print("PASS: bit_crush works")
+
+# Test 8: normalize
+normed = src.normalize()
+assert normed is not None
+assert normed.sample_count == src.sample_count
+print("PASS: normalize works")
+
+# Test 9: reverse
+rev = src.reverse()
+assert rev is not None
+assert rev.sample_count == src.sample_count
+print("PASS: reverse preserves sample count")
+
+# Test 10: slice
+sliced = src.slice(0.1, 0.3)
+assert sliced is not None
+expected_duration = 0.2
+assert abs(sliced.duration - expected_duration) < 0.02, f"Expected ~{expected_duration}s, got {sliced.duration}s"
+print(f"PASS: slice(0.1, 0.3) -> {sliced.duration:.3f}s")
+
+# Test 11: slice out of bounds is safe
+empty = src.slice(0.5, 0.5)  # zero-length
+assert empty.sample_count == 0
+print("PASS: slice with start==end returns empty")
+
+# Test 12: Chaining effects (effects return new buffers)
+chained = src.low_pass(1000).distortion(1.5).normalize()
+assert chained is not None
+assert chained.sample_count > 0
+print("PASS: Chaining effects works")
+
+# Test 13: Effects don't modify original
+orig_count = src.sample_count
+src.pitch_shift(2.0)
+assert src.sample_count == orig_count, "Original should not be modified"
+print("PASS: Effects don't modify original buffer")
+
+# Test 14: pitch_shift with invalid factor raises ValueError
+try:
+    src.pitch_shift(-1.0)
+    assert False, "Should have raised ValueError"
+except ValueError:
+    pass
+print("PASS: pitch_shift with negative factor raises ValueError")
+
+print("\nAll soundbuffer_effects tests passed!")
+sys.exit(0)