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/kernel/devices/platform.zig

@@ -0,0 +1,88 @@
+//! The firmware-agnostic discovery facade.
+//!
+//! The kernel calls `platform.discover()` and gets back a generic `DeviceTree`
+//! without ever naming ACPI or device-tree — the same way it imports `architecture`
+//! without naming x86_64. Which backend runs is decided *at runtime* from what
+//! the bootloader handed us (an ACPI RSDP today, a device-tree blob later),
+//! because a single image — a future ARM kernel especially — may boot under
+//! either firmware. That's a deliberate divergence from `architecture`, which is a
+//! compile-time choice.
+
+const std = @import("std");
+const boot_handoff = @import("boot-handoff");
+const device_model = @import("device-model.zig");
+const acpi = @import("acpi.zig");
+const power = @import("power.zig");
+const devicetree = @import("device-tree.zig");
+
+pub const DeviceTree = device_model.DeviceTree;
+pub const Device = device_model.Device;
+pub const DeviceClass = device_model.DeviceClass;
+pub const Resource = device_model.Resource;
+pub const ResourceKind = device_model.ResourceKind;
+pub const Hal = device_model.Hal;
+pub const PowerInformation = acpi.PowerInformation;
+pub const PlatformInformation = acpi.PlatformInformation;
+pub const RegisterAccess = acpi.RegisterAccess;
+pub const IsoEntry = acpi.IsoEntry;
+pub const Cpu = acpi.Cpu;
+
+/// The FADT power register map discovery extracted (PM1 control, reset register),
+/// for kernel reboot and diagnostics. Sleep-state values are userspace's (S5 is
+/// owned by the ring-3 acpi service), so they are not here.
+pub fn powerInformation() PowerInformation {
+    return acpi.power_information;
+}
+
+/// The scalar firmware facts the architecture layer needs to avoid legacy assumptions
+/// (8259 presence, LAPIC base, PM timer, SPCR UART, IRQ overrides).
+pub fn platformInformation() PlatformInformation {
+    return acpi.platform_information;
+}
+
+/// The usable logical processors discovered during enumeration — one entry per
+/// core danos may schedule on, each carrying the Local APIC ID an SMP wake targets.
+/// `len` is the hardware's degree of parallelism: how many tasks *could* run at the
+/// same instant once the application processors are started. Today only the
+/// bootstrap processor is actually running, so starting the rest is the pending SMP
+/// step (see docs/smp.md). Borrowed from static storage populated by `discover`.
+pub fn cpus() []const Cpu {
+    return acpi.cpu_information.cpus[0..acpi.cpu_information.count];
+}
+
+/// Non-zero only if enumeration found more processors than the static pool holds
+/// (the surplus were dropped from `cpus()`); surfaced so the cap is never silent.
+pub fn cpusDropped() usize {
+    return acpi.cpu_information.dropped;
+}
+
+/// Enumerate hardware into a fresh device tree. `hal` supplies the hardware
+/// primitives the backend needs (MMIO mapping for PCIe configuration space, port I/O for
+/// ACPI registers); pass the architecture implementation. Errors leave nothing to clean up
+/// beyond the tree's own allocations.
+pub fn discover(
+    boot_information: *const boot_handoff.BootInformation,
+    allocator: std.mem.Allocator,
+    hal: Hal,
+) !DeviceTree {
+    var device_tree = try DeviceTree.init(allocator);
+
+    if (boot_information.acpi_rsdp != 0) {
+        const memory_regions = @as([*]const boot_handoff.MemoryRegion, @ptrFromInt(boot_handoff.physicalToVirtual(boot_information.memory_map.regions)))[0..boot_information.memory_map.len];
+        try acpi.discover(boot_information.acpi_rsdp, memory_regions, &device_tree, hal);
+    } else {
+        // No ACPI RSDP. A device-tree boot would parse its blob here; today that
+        // path is a stub, so this reports the machine described itself no way we
+        // understand yet.
+        try devicetree.discover(&device_tree);
+    }
+
+    return device_tree;
+}
+
+/// Restart the machine. Never returns on success; returns only if no reset method
+/// worked (extremely unlikely). Backend-agnostic entry the kernel calls. Soft-off
+/// (S5) is not a kernel operation — the ring-3 acpi service owns it (docs/power.md).
+pub fn reboot(hal: Hal) void {
+    power.reboot(hal);
+}