Add appendable zero-copy PCM chunk streams

[linuxificator]

Prepare AMY PCM playback for realtime producers such as an SD-card task without changing the existing ROM/memory/file PCM behavior.

Design:

• caller-owned fixed descriptor ring; no allocation/copy in the audio render path
• single-producer/single-consumer ownership with FREE/READY/PLAYING/DONE/CANCELLED states
• producer may append chunks while playback is active
• DONE/CANCELLED explicitly tells a ping-pong producer when the underlying memory is safe to overwrite
• an empty unfinished stream waits and can resume when data is appended; finish() turns an empty queue into end-of-stream
• contiguous pcm_load() and existing file/ROM PCM remain on their original path
• one active oscillator per stream; polyphony uses separate stream instances/presets
• sequential streams reject PCM loop modes and do not use the random-access fit stretcher

Tests cover ring ownership, live ping-pong reuse, underrun/resume, stop/cancellation, stream-vs-contiguous preset behavior, and the existing contiguous pcm_load() API.

Build integration includes root Makefile/setup.py plus explicit Godot and Daisy source lists. Arduino builds compile src/** automatically.

CI note: the fork carries upstream's C/C++ matrix. This branch also runs make ctest before make test, so the standalone PCM ownership regression is exercised once GitHub Actions is enabled for the fork.