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/system-requirements.md

@@ -115,7 +115,7 @@ hypervisor configured for UEFI firmware and an xHCI USB controller.
   (`efi.zig:790`, `boot-handoff.zig:149`)
 - **Required ACPI tables:** MADT (interrupt topology), MCFG (PCIe ECAM base),
   FADT (power / PM timer). Optionally consumed: HPET, DMAR, SPCR.
-  (`system/devices/acpi.zig:3`)
+  (`system/kernel/devices/acpi.zig:3`)
 - The loader reads `/system/kernel` off the FAT boot volume, then loads user
   space: a prebuilt `boot\system.img` capsule
   ([system-image.md](system-image.md)) when present, otherwise it walks
@@ -221,7 +221,7 @@ named for reporting only; internal SATA / NVMe / IDE disks have no driver.
 first VT-d DRHD unit and its capabilities are exposed via `PlatformInformation`
 (`iommu_present`, `iommu_base`, `iommu_version`). No DMA-remapping tables are
 programmed and no translation is enforced. An IOMMU is therefore **not required**
-and does not currently constrain devices. (`system/devices/acpi.zig:96`)
+and does not currently constrain devices. (`system/kernel/devices/acpi.zig:96`)
 
 ## What is explicitly NOT supported