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/kernel/log-ring.zig

@@ -161,7 +161,7 @@ test "append/read round trip" {
     defer std.testing.allocator.destroy(ring);
     ring.* = .{};
 
-    _ = ring.append(7, "/system/services/fat", .info, 123, "mounted /mnt/usb", false);
+    _ = ring.append(7, "/system/services/fat", .info, 123, "mounted /volumes/usb", false);
     _ = ring.append(0, "kernel", .raw, 456, "wall clock online", false);
 
     const first = parseAt(ring, ring.tail);
@@ -169,7 +169,7 @@ test "append/read round trip" {
     try std.testing.expectEqual(abi.KlogLevel.info, first.header.level);
     try std.testing.expectEqual(@as(u64, 123), first.header.timestamp_ns);
     try std.testing.expectEqualStrings("/system/services/fat", first.nameSlice());
-    try std.testing.expectEqualStrings("mounted /mnt/usb", first.messageSlice());
+    try std.testing.expectEqualStrings("mounted /volumes/usb", first.messageSlice());
 
     const second = parseAt(ring, first.next(ring.tail));
     try std.testing.expectEqual(@as(u32, 0), second.header.pid);