reorg: move system/devices into system/kernel/devices

With the shared device data (device-abi, pci-class, usb-abi, usb-ids, acpi-ids,
aml) now in library/device/, what remained in system/devices/ was purely
kernel-internal: the firmware-discovery machinery and the rich pointer-based
device model (platform, device-model, acpi, device-tree, power), reached only
through the "platform" module by three kernel files. It belongs with the kernel.

Move it to system/kernel/devices/. A pure relocation: the "platform" module
name is unchanged and every cross-dir dependency is a module import, so only the
one b.path and some comments move. The top-level split is now clean —
system/kernel/ is the kernel, library/device/ the shared device libraries,
system/{drivers,services} the userspace. The runtime /system/devices concept
(the virtual device tree) is unaffected; system/kernel/devices/ is its
implementation.

Also fixes two comment refs that still pointed device-abi at its pre-Wave-1a
home (system/devices/) — it lives at library/device/model/ now.

zig build + test green; smoke, discovery, acpi-parse, acpi-ps2, acpi-report pass.

system/abi.zig

@@ -13,7 +13,7 @@
 //!
 //! This is the *core* contract; the device half — `DeviceDescriptor` and friends, which
 //! also cross this boundary — lives with the device sub-project as [[device-abi]]
-//! (system/devices/device-abi.zig). The loader↔kernel handoff is [[boot-handoff]].
+//! (library/device/model/device-abi.zig). The loader↔kernel handoff is [[boot-handoff]].
 
 /// Page size every `mmap`/`munmap` grant and the boot memory map are measured in.
 /// 4 KiB on every architecture danos targets so far. Part of the ABI because the

system/boot-handoff.zig

@@ -5,7 +5,7 @@
 //!
 //! Both binaries import this as the `boot-handoff` module, so the layout is defined
 //! in exactly one place. **User space never sees this** — the kernel↔user contract is
-//! [[abi]] (system/abi.zig); device types are [[device-abi]] (system/devices/device-abi.zig).
+//! [[abi]] (system/abi.zig); device types are [[device-abi]] (library/device/model/device-abi.zig).
 
 const std = @import("std");