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/README.md

@@ -238,9 +238,9 @@ system/          → /system   danos's own internals (the self-representation)
   parameters.zig  initial-ramdisk.zig   shared contracts
   kernel/                     IPC, memory, scheduling, the VFS root, the private syscall dispatch
     architecture/x86_64/      the `architecture` module (never named by generic code)
-    devices/                  the kernel-internal device model + firmware discovery
-                              (device-model, platform, acpi, device-tree, power) — the
-                              implementation of the device tree /system/devices reflects
+    devices-broker.zig        the syscall-facing device table
+    platform.zig acpi.zig fdt.zig device-model.zig   firmware discovery + the kernel's
+                              device model — the implementation of what /system/devices reflects
   drivers/     pci-bus/ ps2-bus/ usb-xhci-bus/   one sub-project per driver → /system/drivers
   services/    init/ fat/ device-manager/   system servers → /system/services (fat/ holds
                               fat.zig, engine.zig, on-disk.zig)
@@ -300,7 +300,7 @@ exception in [coding-standards.md](coding-standards.md) applies to that seam.
 | VFS root: mount table + kernel-served nodes (`fs_resolve`/`fs_node`); wire protocol in `library/protocol/vfs/vfs-protocol.zig` | `system/kernel/vfs.zig` |
 | Device tree + claim capability + `device_register` containment | `system/kernel/devices-broker.zig` |
 | IRQ-as-IPC: routing a device interrupt to a driver's endpoint | `system/kernel/irq.zig` |
-| Hardware discovery (ACPI/device tree) behind one neutral device model | `system/kernel/devices/` |
+| Hardware discovery (ACPI/device tree) behind one neutral device model | `system/kernel/platform.zig` |
 | Framebuffer text console (mirrors to serial) | `system/kernel/console.zig` |
 | In-kernel test cases | `system/kernel/tests.zig` |
 | Arch-specific kernel code (`halt`, GDT/IDT/TSS, exception + interrupt stubs, page tables, APIC/IO-APIC/timer, serial, linker script) | `system/kernel/architecture/x86_64/` |