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.

build.zig

@@ -278,7 +278,7 @@ pub fn build(b: *std.Build) void {
     });
     // The devices sub-project's public interface (the flat wire types), exposed as
     // its own module like vfs-protocol — importable by user space, unlike the
-    // kernel-internal device model it also feeds (system/kernel/devices/device-model.zig).
+    // kernel-internal device model it also feeds (system/kernel/device-model.zig).
     const device_abi_module = b.addModule("device-abi", .{
         .root_source_file = b.path("library/device/model/device-abi.zig"),
     });
@@ -351,9 +351,9 @@ pub fn build(b: *std.Build) void {
     // "platform" and asks it to enumerate hardware into a backend-neutral device
     // tree, never naming ACPI (or, later, device-tree) — the same discipline the
     // architecture module applies to CPU code. The backend is selected at runtime from
-    // the boot handoff (see system/kernel/devices/platform.zig).
+    // the boot handoff (see system/kernel/platform.zig).
     const platform_module = b.addModule("platform", .{
-        .root_source_file = b.path("system/kernel/devices/platform.zig"),
+        .root_source_file = b.path("system/kernel/platform.zig"),
         .imports = &.{
             .{ .name = "boot-handoff", .module = boot_handoff_module }, // BootInformation (carries the ACPI RSDP), physicalToVirtual
             .{ .name = "abi", .module = abi_module }, // acpi.zig works in page_size units