feat(core): support PCM through the shared audio stream interface

[opencode-agent]

Summary

Add PCM as a format of the existing audio stream interface, from TypeScript through the native ABI. This is an updated version of #1480 on an upstream branch because the bot cannot push to the original fork branch.

const stream = await audio.playStream(source, {
  format: "pcm",
  sampleFormat: "f32le",
  sampleRate: 44100,
  channels: 2,
})
await stream.closed

• One native audioCreateStream / audioWriteStream / audioEndStream contract. Creation takes the format plus its input rate/channels; writes accept and return byte counts. The native export file is unchanged from main.
• TypeScript uses the same source pump, backpressure, readiness, controls, cancellation, and completion for PCM, MP3, and FLAC. No PCM-specific stream class, native method family, sample conversion loop, or additional worker.
• Native PCM ingestion handles unaligned little-endian input, partial frames, and finite-sample validation in bounded batches. Miniaudio converts directly into the existing stereo mixer ring.
• Remove unused pause/clear controls, sound reconstruction, and duplicated PCM counters. Preserve the EOF-tail fix so fractional downsampling latency cannot discard short audio.
• Update the byte-contract/ABI, fragmentation, ownership, cancellation, fairness, resampling, and EOF regressions, plus docs and the generated-tone example. PCM reconnect remains explicitly unsupported.

Validation status

• Native PCM tests: 8 passed; broader native stream tests: 41 passed.
• Shared FFI/ABI tests: 26 passed. Typecheck, formatting, lint, documentation validation, and independent code review passed.
• Full native suite: 2,164 passed, 8 skipped, one existing X11 socket-directory failure, previously reproduced on the original PR head. Clipboard source is unchanged.
• The local ReleaseFast rebuild is still compiling. Bun/Node runtime revalidation and packed-distribution checks against the rebuilt library are pending. Earlier runtime attempts loaded the stale pre-refactor library and are not valid results for this revision; no full-runtime pass is claimed yet.

Requested by: @neriousy (Filip via Slack); simplification requested by @kommander (Sebastian via Slack)