build: the unix paths retire — configuration, logs, and volumes move into the danos tree

/etc/init.csv and /etc/devices.csv become /system/configuration/*.csv (the
repo's etc/ moves to system/configuration/, mirroring the runtime tree),
/var/log becomes /system/logs, and /mnt/usb becomes /volumes/usb. The
kernel VFS gains a carve-out so FAT may serve exactly /system/configuration
and /system/logs beneath the initrd-backed /system while /system and /test
themselves stay unshadowable; FAT's single /var mount splits into those two
rewritten mounts. The kvfs readdir check learns /system's third child and
the ramdisk spawn sweep skips the configuration tree.

Suite 106/106.

system/abi.zig

@@ -295,7 +295,7 @@ pub const ServiceId = enum(u32) {
     power = 5, // system power: events (button, lid, battery) + shutdown (docs/power.md; domain-named per docs/discovery.md — the acpi service registers it on x86, a PSCI service will on ARM)
     usb_bus = 6, // the xHCI host-controller driver's transfer endpoint; USB class drivers look it up and `callCap`-open their device to get a private per-device transfer channel (docs/driver-model.md)
     block = 7, // a block-device driver (USB mass storage today): read/write of fixed-size blocks, the storage a filesystem sits on
-    fat = 8, // the FAT filesystem server; the VFS mounts it and forwards paths under its mount point (/mnt/usb) to it
+    fat = 8, // the FAT filesystem server; the VFS mounts it and forwards paths under its mount point (/volumes/usb) to it
     display = 9, // the display service: owns the framebuffer, composites a layer stack, presents frames (docs/display.md)
     shared_memory_test = 10, // the shared-memory test server (V2): a client passes it a shared-memory capability, it maps + verifies (docs/display-v2.md)
     scanout = 11, // a native scanout driver (virtio-gpu): the compositor finds it here to upgrade off the GOP framebuffer (docs/display-v2.md)