Moves the WebAudio→engine bus — ring, producer-side linear resampler,
prefill gate, fill clamp, metrics — verbatim into
src/audio/engine/RendererBus.h. AudioEngine keeps thin facades
(setRendererBus/pushRendererAudio/pullRendererBus/getRendererBusMetrics) so
the NodeAddon surface is unchanged. JUCE-free: pull() takes raw channel
pointers, which is what lets tests/engine_units drive the resampler
continuity, prime/underflow/clamp, and metrics cases without a device.
The control-thread readIndex write on disable (deep-read §4) is preserved
verbatim and marked; its flush-flag fix lands as the phase-8 commit.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
