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.

build.zig

@@ -278,7 +278,7 @@ pub fn build(b: *std.Build) void {
     });
     // The devices sub-project's public interface (the flat wire types), exposed as
     // its own module like vfs-protocol — importable by user space, unlike the
-    // kernel-internal device model it also feeds (system/devices/device-model.zig).
+    // kernel-internal device model it also feeds (system/kernel/devices/device-model.zig).
     const device_abi_module = b.addModule("device-abi", .{
         .root_source_file = b.path("library/device/model/device-abi.zig"),
     });
@@ -351,9 +351,9 @@ pub fn build(b: *std.Build) void {
     // "platform" and asks it to enumerate hardware into a backend-neutral device
     // tree, never naming ACPI (or, later, device-tree) — the same discipline the
     // architecture module applies to CPU code. The backend is selected at runtime from
-    // the boot handoff (see system/devices/platform.zig).
+    // the boot handoff (see system/kernel/devices/platform.zig).
     const platform_module = b.addModule("platform", .{
-        .root_source_file = b.path("system/devices/platform.zig"),
+        .root_source_file = b.path("system/kernel/devices/platform.zig"),
         .imports = &.{
             .{ .name = "boot-handoff", .module = boot_handoff_module }, // BootInformation (carries the ACPI RSDP), physicalToVirtual
             .{ .name = "abi", .module = abi_module }, // acpi.zig works in page_size units