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.

build/qemu.zig

@@ -42,8 +42,8 @@ pub fn addRunSteps(b: *std.Build, fat_image_serial: std.Build.LazyPath) void {
     const vars_out = vars_copy.addOutputFileArg("OVMF_VARS.4m.fd");
 
     // Capture the guest's serial0 (danos's machine-readable log) to the qemu-test
-    // scratch area — a dev/host artifact, kept out of the FHS boot volume we mount.
-    // (/var/log/system is reserved for the kernel's own logging system later.) One
+    // scratch area — a dev/host artifact, kept out of the boot volume we mount.
+    // (/system/logs on the volume belongs to the guest's own logger.) One
     // timestamped file per run.
     const log_dir = b.fmt("{s}/qemu-test", .{b.install_path});
     const make_log_dir = b.addSystemCommand(&.{ "mkdir", "-p", log_dir });