Harden ARM64 AHCI exec reads against signal interruption

[ryanbreen]

Summary

• root-causes the relaxed ARM64 init failure to signal-interrupted AHCI completion waits during exec reads
• adds an explicit uninterruptible Completion wait for already-issued hardware commands and uses it for AHCI slot-0 waits
• removes the F19 init serialization workaround: bsshd starts before the service wave, and the per-service/bounce yield+nanosleep delays are gone

Evidence

• Reproduction before fix, with init serialization removed: /bin/bwm exec failed with AHCI: interrupted immediately after xhci_counters exited/SIGCHLD arrived.
  • turn336-artifacts/relaxed-prefix.serial.log lines 425-428
• Fixed fresh Parallels ARM64 boot: bsshd, heartbeat, bwm, telnetd, and bounce all start under relaxed ordering.
  • turn336-artifacts/fixed-relaxed.serial.log lines 381-487
• Live fixed no-build replay reached ~79s uptime with sustained heartbeat and 82 bwm FPS samples in range 114-191.
  • turn336-artifacts/fixed-relaxed-live.serial.log
  • turn336-artifacts/fixed-relaxed-live.screenshot.png shows Bounce on the BWM desktop
• Clean build check: ./run.sh --parallels --test 70 produced no compile-stage warning/error lines via grep -E "^(warning|error)" turn336-artifacts/fixed-relaxed-run.log.
• Negative checks: no AHCI: interrupted, AHCI timeout, panic, lockup, SCHED_RESCUE, or VIRTGPU_FAIL in fixed serial logs.

Notes

• cargo fmt is blocked by pre-existing trailing whitespace in tests/shared_qemu.rs; the touched files were formatted directly with rustfmt.

Fixes breenix-4yu