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/acpi.md

@@ -19,10 +19,10 @@ UEFI configuration table
 BootInformation.acpi_rsdp   (u64, in the loader↔kernel handoff)   system/boot-handoff.zig
    │  the kernel forwards the whole BootInformation
    ▼
-platform.discover(boot_information, …)                                  system/kernel/devices/platform.zig
+platform.discover(boot_information, …)                                  system/kernel/platform.zig
    │  reads boot_information.acpi_rsdp, hands it to the ACPI backend
    ▼
-acpi.discover(rsdp_phys, …)                                      system/kernel/devices/acpi.zig
+acpi.discover(rsdp_phys, …)                                      system/kernel/acpi.zig
    │  dereferences the RSDP, reads the pointer it contains
    ▼
 RSDP ──(a field in the struct)──► RSDT / XSDT ──► SDTs (MADT, MCFG, FADT, HPET, DSDT…)