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/services/fat/engine.zig

@@ -73,7 +73,7 @@ const entries_per_sector = sector_size / @sizeOf(on_disk.DirectoryEntry); // 16
 // A small write-through cache of single-sector (metadata) accesses: FAT sectors,
 // directory sectors, and directory-entry writebacks. Its payoff is repeated scans
 // — resolving many paths under the same directory (a logging burst opening dozens
-// of files under /var/log/<stamp>/) re-reads the same directory and FAT sectors,
+// of files under /system/logs/<stamp>/) re-reads the same directory and FAT sectors,
 // which now come from RAM instead of a USB round trip each. Bulk file data (the
 // multi-sector run path) bypasses the cache — it is large and not re-read — and a
 // run write invalidates any overlapping cached sector to stay coherent.