The power button, in ring 3: SCI bound, fixed event published (M21.1)

The kernel publishes the FADT as one more acpi-tables memory resource
(tagged by its intact FACP header — the AML blobs are header-stripped);
the acpi service reads the PM1 event/control and GPE register ports from
that copy, so the kernel's own FADT parse is untouched. A power-protocol
module (ServiceId.power = 5, domain-named so an ARM PSCI service can serve
the same id) carries subscribe / shutdown / events.

The acpi service converts to runtime.service.run — device discovery, the
.power protocol, and the SCI notification all fold into one loop. At
startup it enables ACPI mode if SCI_EN is clear (the SMI dance), binds the
SCI (found as the node's len-1 irq resource, distinct from the broad
window), and sets PWRBTN_EN. On the SCI it reads PM1_STS, clears
PWRBTN_STS write-1, logs the press, publishes power_button to
subscribers, and always acks. The power-button scenario proves it with a
real QMP system_powerdown injected mid-run through the M21.0 channel.

test/qemu_test.py

@@ -292,6 +292,15 @@ CASES = [
                r"device-manager: spawned ps2-bus[\s\S]*"
                r"ps2-bus: keyboard driver attached",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
+    # M21.1: the SCI + power button. Boot the manager (which spawns the acpi
+    # service); ~4s in, QMP system_powerdown raises the ACPI power-button fixed
+    # event; the service's SCI handler must log the press (docs/m21-plan.md).
+    {"name": "power-button",
+     "smp":    4,
+     "timeout": 60,
+     "qmp_after": {"delay": 4, "command": "system_powerdown"},
+     "expect": r"power: button pressed",
+     "fail":   r"DANOS-TEST-RESULT: FAIL"},
     # M20.2: the acpi service evaluates _CRS/_STA in ring 3 and registers +
     # reports its _HID devices — the two PS/2 nodes must appear with resources
     # (keyboard: io 0x60/0x64 + IRQ = 3; mouse: IRQ = 1) (docs/m19-m20-plan.md).
@@ -342,6 +351,7 @@ CASES = [
     # The initial_ramdisk: the loader ferries a bundle of user binaries; the kernel parses
     # it and spawns each as a ring-3 process (here the VFS-server stub heartbeats).
     {"name": "initial-ramdisk",
+     "timeout": 60,  # the acpi service's boot-time SCI setup can push the marker past 30s under load
      "expect": r"DANOS-TEST-RESULT: PASS",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
     # The user-space VFS: a client opens/writes/reads a file through the rt file