feat(au): complete audio and MIDI support

[unohee]

What

• complete the AUv2 audio render, lifecycle, property, bypass, tail, latency, state, and Cocoa editor paths
• implement AUv2 MIDI input (MIDIEvent, SysEx, StartNote, StopNote) with bounded queues and sample offsets
• implement AUv2 MIDI output callback/property support without locks or allocations on the render path
• turn the sine example into a real aumu instrument and add a LaunchServices-based macOS host smoke test
• add pinned CodeQL and optimize the existing build, test, and docs workflows
• pin the compatible public Intrect-io/iced_baseview revision needed by the existing baseview fork

Why

The fork had an incomplete AU wrapper: host audio behavior and several AU properties/lifecycle contracts were missing, MIDI selectors and output callbacks were not wired, and the existing CLI-only validation could not prove component discovery or render behavior on current macOS. The iced editor also depended on an iced_baseview revision incompatible with the pinned baseview ABI.

Impact

• AU instruments can receive MIDI, render audio, and emit MIDI through the legacy AUv2 host contract.
• MIDI queues are bounded, timing is clamped per render block, and callback replacement does not block the audio thread.
• The smoke harness verifies the packaged component through an actual .app host and restores any pre-existing installed component.
• CI action references are SHA-pinned, cache keys are deterministic, and CodeQL covers Actions and Rust.

Validation

• cargo test --locked --workspace --features 'simd,standalone,zstd'
• cargo test --locked --lib --features 'au,vst3,zstd' — 119 passed
• release library tests — 108 passed
• no-default-feature build and pinned-nightly docs build passed
• macOS universal package build passed for all 12 example packages; AU binaries verified as arm64 + x86_64
• scripts/au_midi_smoke.sh passed component discovery, bus layout, pre-note silence, note-on audio, note-off, SysEx, Start/StopNote, MIDI output echo, LastRenderError, and infinite tail checks
• local Actions CodeQL: 0 findings
• local Rust CodeQL: no security findings and no changed-path findings (remaining results are repository-wide unused-variable quality findings)
• OpenSwarm whole-change review: APPROVE

Stack

This draft targets aud-747-audit-fixes (PR #2) so the AUD-834 diff stays isolated. Retarget it to master after PR #2 lands.

Linear: AUD-834

[unohee]

🔍 Fresh review of 0371317 (openswarm pr review --fresh)

✓ Decision: APPROVE
No material correctness or production-wiring defect found. AU MIDI selector dispatch, queueing, callback output, bundle metadata, example export, and macOS smoke-test wiring are present and mutually consistent.

[unohee]

🔍 Fresh review of 63f2913 (openswarm pr review --fresh)

✎ Decision: REVISE
The AU MIDI-output callback storage has an unbounded per-instance memory leak when a host replaces its callback. Other reviewed MIDI production wiring is present, including selector dispatch and the macOS smoke host.

[unohee]

🔍 Fresh review of 971d26c (openswarm pr review --fresh)

✓ Decision: APPROVE
The AU MIDI path is wired from MusicDevice selectors through bounded render-time queues into ProcessContext and back through the AU MIDI-output callback. The smoke host exercises component discovery, rendering, basic MIDI I/O, SysEx acceptance, extended start/stop notes, LastRenderError, and TailTime. No concrete correctness defect found.