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/PySoundBuffer.h

@@ -0,0 +1,123 @@
+#pragma once
+#include "Common.h"
+#include "Python.h"
+#include "McRFPy_Doc.h"
+#include <vector>
+#include <memory>
+#include <string>
+#include <cstdint>
+
+// Forward declarations
+struct SfxrParams;
+
+// Core audio data container - holds authoritative sample data
+class SoundBufferData : public std::enable_shared_from_this<SoundBufferData>
+{
+public:
+    std::vector<int16_t> samples;
+    unsigned int sampleRate = 44100;
+    unsigned int channels = 1;
+
+    // Optional sfxr params (set when created via sfxr synthesis)
+    std::shared_ptr<SfxrParams> sfxrParams;
+
+    // Lazy sf::SoundBuffer rebuild
+    sf::SoundBuffer sfBuffer;
+    bool sfBufferDirty = true;
+
+    SoundBufferData() = default;
+    SoundBufferData(std::vector<int16_t>&& s, unsigned int rate, unsigned int ch)
+        : samples(std::move(s)), sampleRate(rate), channels(ch), sfBufferDirty(true) {}
+
+    // Rebuild sf::SoundBuffer from samples if dirty
+    sf::SoundBuffer& getSfBuffer() {
+        if (sfBufferDirty && !samples.empty()) {
+            sfBuffer.loadFromSamples(samples.data(), samples.size(), channels, sampleRate);
+            sfBufferDirty = false;
+        }
+        return sfBuffer;
+    }
+
+    float duration() const {
+        if (sampleRate == 0 || channels == 0 || samples.empty()) return 0.0f;
+        return static_cast<float>(samples.size()) / static_cast<float>(channels) / static_cast<float>(sampleRate);
+    }
+};
+
+// Python object wrapper
+typedef struct {
+    PyObject_HEAD
+    std::shared_ptr<SoundBufferData> data;
+} PySoundBufferObject;
+
+// Python type methods/getset declarations
+namespace PySoundBuffer {
+    // tp_init, tp_new, tp_repr
+    int init(PySoundBufferObject* self, PyObject* args, PyObject* kwds);
+    PyObject* pynew(PyTypeObject* type, PyObject* args, PyObject* kwds);
+    PyObject* repr(PyObject* obj);
+
+    // Class methods (factories)
+    PyObject* from_samples(PyObject* cls, PyObject* args, PyObject* kwds);
+    PyObject* tone(PyObject* cls, PyObject* args, PyObject* kwds);
+    PyObject* sfxr(PyObject* cls, PyObject* args, PyObject* kwds);
+    PyObject* concat(PyObject* cls, PyObject* args, PyObject* kwds);
+    PyObject* mix(PyObject* cls, PyObject* args, PyObject* kwds);
+
+    // Instance methods (DSP - each returns new SoundBuffer)
+    PyObject* pitch_shift(PySoundBufferObject* self, PyObject* args);
+    PyObject* low_pass(PySoundBufferObject* self, PyObject* args);
+    PyObject* high_pass(PySoundBufferObject* self, PyObject* args);
+    PyObject* echo(PySoundBufferObject* self, PyObject* args);
+    PyObject* reverb(PySoundBufferObject* self, PyObject* args);
+    PyObject* distortion(PySoundBufferObject* self, PyObject* args);
+    PyObject* bit_crush(PySoundBufferObject* self, PyObject* args);
+    PyObject* normalize(PySoundBufferObject* self, PyObject* args);
+    PyObject* reverse(PySoundBufferObject* self, PyObject* args);
+    PyObject* slice(PySoundBufferObject* self, PyObject* args);
+    PyObject* sfxr_mutate(PySoundBufferObject* self, PyObject* args);
+
+    // Properties
+    PyObject* get_duration(PySoundBufferObject* self, void* closure);
+    PyObject* get_sample_count(PySoundBufferObject* self, void* closure);
+    PyObject* get_sample_rate(PySoundBufferObject* self, void* closure);
+    PyObject* get_channels(PySoundBufferObject* self, void* closure);
+    PyObject* get_sfxr_params(PySoundBufferObject* self, void* closure);
+
+    extern PyMethodDef methods[];
+    extern PyGetSetDef getsetters[];
+}
+
+// Helper: create a new PySoundBufferObject wrapping given data
+PyObject* PySoundBuffer_from_data(std::shared_ptr<SoundBufferData> data);
+
+namespace mcrfpydef {
+    inline PyTypeObject PySoundBufferType = {
+        .ob_base = {.ob_base = {.ob_refcnt = 1, .ob_type = NULL}, .ob_size = 0},
+        .tp_name = "mcrfpy.SoundBuffer",
+        .tp_basicsize = sizeof(PySoundBufferObject),
+        .tp_itemsize = 0,
+        .tp_repr = PySoundBuffer::repr,
+        .tp_flags = Py_TPFLAGS_DEFAULT,
+        .tp_doc = PyDoc_STR(
+            "SoundBuffer(filename: str)\n"
+            "SoundBuffer.from_samples(data: bytes, channels: int, sample_rate: int)\n"
+            "SoundBuffer.tone(frequency: float, duration: float, waveform: str = 'sine', ...)\n"
+            "SoundBuffer.sfxr(preset: str, seed: int = None)\n\n"
+            "Audio sample buffer for procedural audio generation and effects.\n\n"
+            "Holds PCM sample data that can be created from files, raw samples,\n"
+            "tone synthesis, or sfxr presets. Effect methods return new SoundBuffer\n"
+            "instances (copy-modify pattern).\n\n"
+            "Properties:\n"
+            "    duration (float, read-only): Duration in seconds.\n"
+            "    sample_count (int, read-only): Total number of samples.\n"
+            "    sample_rate (int, read-only): Samples per second (e.g. 44100).\n"
+            "    channels (int, read-only): Number of audio channels.\n"
+            "    sfxr_params (dict or None, read-only): sfxr parameters if sfxr-generated.\n"
+        ),
+        .tp_methods = PySoundBuffer::methods,
+        .tp_getset = PySoundBuffer::getsetters,
+        .tp_init = (initproc)PySoundBuffer::init,
+        .tp_new = PySoundBuffer::pynew,
+    };
+}