perf(macos): reduce audio and prefetch UI stalls

[tcconnally]

Summary

• Replace synchronous libmpv time-pos IPC during lazy audio relock with the existing event-thread-maintained position cache.
• Defer loadfile append prefetch one Qt event-loop turn, keeping next/advance handlers short while preserving the playlist warmup path.
• Add explicit audio-arm timings plus a macOS audio-focused soak launcher/JSONL telemetry artifacts.

Evidence

The M5 five-minute soak recorded a 210 ms cell._toggle_mute stall and repeated 170–246 ms wall._arm_prefetch / wall.next_video GUI stalls. VideoToolbox remained active on every cell; cache starvation is separately visible and remains out of scope for this targeted UI-latency change.

Validation

• python3 tests/run_all.py — PASS
• bash -n soak_wall.sh — PASS
• New regression contracts cover cached audio relock and deferred prefetch scheduling.

Rerun

Run ./soak_wall.sh 5 on the M5. Compare AUDIO arm, PERF slow slot, PERF loop stall, and freeze counters to the baseline.