feat: ACPI shutdown hardening with timeout, PM1b retry, and keyboard reset fallback

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

recipes/core/base/P4-acpi-shutdown-hardening.patch

@@ -0,0 +1 @@
+../../../local/patches/base/P4-acpi-shutdown-hardening.patch

recipes/core/base/recipe.toml

@@ -12,6 +12,7 @@ patches = [
     "P3-usbhidd-hardening.patch",
     "P3-init-colored-output.patch",
     "P4-logd-persistent-logging.patch",
+    "P4-acpi-shutdown-hardening.patch",
 ]
 
 [build]