Audio fixes: gain() DSP effect, sfxr phase wrap, SDL2 backend compat

- SoundBuffer.gain(factor): new DSP method for amplitude scaling before
  mixing (0.5 = half volume, 2.0 = double, clamped to int16 range)
- Fix sfxr square/saw waveform artifacts: phase now wraps at period
  boundary instead of growing unbounded; noise buffer refreshes per period
- Fix PySound construction from SoundBuffer on SDL2 backend: use
  loadFromSamples() directly instead of copy-assign (deleted on SDL2)
- Add Image::create(w, h, pixels) overload to HeadlessTypes and
  SDL2Types for pixel data initialization
- Waveform test suite (62 lines)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/PySound.cpp

@@ -18,7 +18,10 @@ PySound::PySound(std::shared_ptr<SoundBufferData> bufData)
     : source("<SoundBuffer>"), loaded(false), bufferData(bufData)
 {
     if (bufData && !bufData->samples.empty()) {
-        buffer = bufData->getSfBuffer();
+        // Rebuild the sf::SoundBuffer from sample data directly
+        // (avoids copy-assign which is deleted on SDL2 backend)
+        buffer.loadFromSamples(bufData->samples.data(), bufData->samples.size(),
+                               bufData->channels, bufData->sampleRate);
         sound.setBuffer(buffer);
         loaded = true;
     }