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.

library/device/model/device-abi.zig

@@ -6,7 +6,7 @@
 //! by name, and neither reaches into the other's files.
 //!
 //! It is also the **single source of truth** for `DeviceClass` and `ResourceKind`:
-//! the kernel's rich, pointer-based device tree (system/devices/device-model.zig,
+//! the kernel's rich, pointer-based device tree (system/kernel/devices/device-model.zig,
 //! which user space must never import) re-exports these, so the enum that a driver
 //! matches on and the enum the kernel classifies with are the *same* type — no
 //! hand-kept "mirror in order" to drift. The core kernel↔user ABI is [[abi]]; the