fix(audio): Render audio in small chunks to fix timing gaps

Fixed issue where drum patterns had silence gaps between cycles. The problem
was that audio_render_ahead was rendering audio in large chunks (up to 200ms),
causing the synth internal time to become desynchronized from tracker events.

Problem:
- audio_render_ahead checked buffer fullness, then rendered large chunk
- First call: buffer empty, render 200ms, synth advances by 200ms
- Next 12 calls: buffer > 100ms, do not render, synth state frozen
- Call 13: buffer < 100ms, render more, but tracker triggered events in between
- Events triggered between render calls ended up at wrong synth time position
- Result: Silence gaps between patterns

Solution:
- Changed audio_render_ahead to render in small incremental chunks
- Chunk size: one frame worth of audio (~16.6ms @ 60fps)
- Loop until buffer reaches target lookahead (200ms)
- Synth now advances gradually, staying synchronized with tracker

Result: Synth time stays synchronized with tracker event timing, no gaps.

Testing: All 17 tests pass (100%)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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