kernel: flatten the device/discovery code, merge power into acpi, rename fdt

Follow-up cleanup of the just-moved kernel device code:

- power.zig -> acpi.zig. Its reboot() is built entirely on the FADT reset
  register (acpi.power_information) plus the legacy 0xCF9/8042 fallbacks — it is
  ACPI reboot, so it becomes acpi.reboot (the "P" in ACPI). platform.reboot
  still delegates; soft-off/S5 stays the ring-3 acpi service's job as before.

- device-tree.zig -> fdt.zig. It is a discovery *backend* (the ARM/FDT parser,
  a sibling of acpi.zig), not part of the model — renaming it to its actual
  subject kills the confusing device-tree / device-model DeviceTree name clash
  and makes acpi.zig + fdt.zig read as the two parallel backends.

- Flatten: device-model.zig, acpi.zig, fdt.zig, platform.zig move out of the
  system/kernel/devices/ subdir up into system/kernel/, joining devices-broker.zig
  (already flat). The kernel dir is a flat pile by convention (only architecture/
  is a subdir), so the subdir — and its poor "devices" name — is gone.

Pure restructure; "platform" module name unchanged, all cross-file deps are
relative siblings that moved together. zig build + test green; smoke, discovery,
acpi-parse, acpi-ps2, acpi-report, power-button, orderly-shutdown, reboot pass.

docs/system-requirements.md

@@ -115,7 +115,7 @@ hypervisor configured for UEFI firmware and an xHCI USB controller.
   (`efi.zig:790`, `boot-handoff.zig:149`)
 - **Required ACPI tables:** MADT (interrupt topology), MCFG (PCIe ECAM base),
   FADT (power / PM timer). Optionally consumed: HPET, DMAR, SPCR.
-  (`system/kernel/devices/acpi.zig:3`)
+  (`system/kernel/acpi.zig:3`)
 - The loader reads `/system/kernel` off the FAT boot volume, then loads user
   space: a prebuilt `boot\system.img` capsule
   ([system-image.md](system-image.md)) when present, otherwise it walks
@@ -221,7 +221,7 @@ named for reporting only; internal SATA / NVMe / IDE disks have no driver.
 first VT-d DRHD unit and its capabilities are exposed via `PlatformInformation`
 (`iommu_present`, `iommu_base`, `iommu_version`). No DMA-remapping tables are
 programmed and no translation is enforced. An IOMMU is therefore **not required**
-and does not currently constrain devices. (`system/kernel/devices/acpi.zig:96`)
+and does not currently constrain devices. (`system/kernel/acpi.zig:96`)
 
 ## What is explicitly NOT supported