Phase 3: thread-safety & resource fixes (atomic stream state, JACK/ASIO/OSS)

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Overview

Phase 3 of the audit-and-hardening work, building on the merged #1. These are thread-safety and resource-management fixes in the realtime/teardown paths of the backends, plus the core stream_.state change. Findings and status are tracked in AUDIT.md.

All 16 CI jobs pass on this branch (run #5). The stream_.state change is portable and is additionally covered by the ASan/UBSan and StrictWarnings (g++ & clang++) gates; the backend changes are compile-verified across the matrix (Linux ALSA/Pulse/JACK/OSS, macOS/CoreAudio, MinGW WASAPI/ASIO/DirectSound) but — lacking the target hardware — were not runtime-tested. Please review the backend changes with that in mind.

Changes

• core — stream_.state is now std::atomic<StreamState> (H-5). It's read by the realtime callback thread and written by the control thread (e.g. stopStream) without the stream mutex. All ~180 access sites are plain load/store (no address-of, switch, or arithmetic), so the type change is transparent while removing the data race.
• JACK — closeStream() now always calls jack_deactivate() before unregistering ports / freeing the handle (was only when state == RUNNING), closing the window where the realtime xrun callback ran against freed ports (H-4). The duplex INPUT error path no longer tears down the client/handle shared with the already-open OUTPUT pass — it only frees the handle when this call allocated it (H-2).
• ASIO — the process-global asioCallbackInfo is cleared in closeStream() and NULL-checked in bufferSwitch(), sampleRateChanged() and asioMessages(), so a late driver callback can't dereference destroyed stream state (H-3).
• OSS — OssHandle::id[] uses -1 (not 0) as the "unused" sentinel and the teardown paths close only descriptors >= 0; previously fd 0 (a valid descriptor) was treated as "no device" and leaked (M-1).

Deferred (see AUDIT.md)

Remaining low-severity items: the ASIO single-instance streamOpen check-then-set race (ASIO is effectively single-driver), and assorted ignored return codes (ALSA snd_pcm_drop, OSS ioctls, WASAPI/DS CreateEvent/CoCreateInstance) and non-predicate-loop drain handshakes.

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