Device manager (increment 1): discover + match

The device manager is the ring-3 process that turns the device tree into a running
system — the udev-analog. It is mechanism-vs-policy done right: the kernel
enumerates the hardware and enforces the claim capability; this decides which
driver serves which device, using no special privilege (the same device_enumerate
any process could call).

This first increment does the discovery + matching half: system/services/
device-manager enumerates /system/devices, matches each device to a driver by
class (a small static policy table), and logs the decision — finding the HPET
(a timer) and deciding `hpet` serves it. It does not spawn yet: spawning needs a
`system_spawn` system call (the kernel spawns every initial-ramdisk binary in a
loop today), which is the next increment. New `device-manager` test; suite 36/36
plus host tests.

docs/README.md

@@ -152,7 +152,7 @@ system/          → /system   danos's own internals (the self-representation)
     architecture/x86_64/      the `architecture` module (never named by generic code)
   devices/                    the device model /system/devices reflects (+ aml/)
   drivers/     hpet/ bus/   one sub-project per driver → /system/drivers
-  services/    init/ vfs/     system servers → /system/services (vfs/ holds
+  services/    init/ vfs/ device-manager/   system servers → /system/services (vfs/ holds
                               vfs.zig, vfs-test.zig, protocol.zig)
 library/         → /lib       libraries, one sub-directory each
   runtime/                    the danos-native runtime — the stable application ABI
@@ -193,7 +193,7 @@ appears in the private-ABI path.
 | Arch-specific kernel code (`halt`, GDT/IDT/TSS, exception + interrupt stubs, page tables, APIC/IO-APIC/timer, serial, linker script) | `system/kernel/architecture/x86_64/` |
 | danos-native runtime (`runtime`): syscall wrappers, heap, IPC, device access — the stable application ABI | `library/runtime/` |
 | POSIX/C compatibility (`posix`): unistd, stdio — the one place POSIX names are allowed | `library/posix/` |
-| System services (init, the VFS server + its `protocol` module) | `system/services/` |
+| System services (init, the VFS server + `protocol`, the device-manager) | `system/services/` |
 | Device drivers, one sub-project each (`hpet` leaf driver, `bus` bus driver) | `system/drivers/` |
 | Build + `run-x86-64` (QEMU/OVMF) | `build.zig` |
 | QEMU integration test harness | `test/qemu_test.py` |