Phase A1 from BOOT-PROCESS-AUDIT. The ACPI shutdown path now:
- Validates PM1a port is non-zero before writing
- Waits 3 seconds for power-off, then retries with PM1b+SLEEP_EN
- Falls back to keyboard controller reset (0x64=0xFE) on failure
- Handles SLP_TYPb correctly
- Removes fragile Pio::new()+write() without validation
