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.

docs/power.md

@@ -92,7 +92,7 @@ fails rather than hangs.
 `acpi-tables` node ([discovery.md](discovery.md)) already put the PM1 control
 ports in the acpi service's hands, so writing S5 from ring 3 is something it
 could physically already do; formalizing it as a protocol operation added a
-contract, not authority. The kernel's `system/devices/power.zig` keeps only
+contract, not authority. The kernel's `system/kernel/devices/power.zig` keeps only
 **reboot** (the FADT reset register plus the legacy fallbacks, which need no AML);
 it has no poweroff path at all — S5 is not a kernel operation.