D4: the machine makes directories too

mkdir and rmdir are the machine's own now, and a file goes where its path says
rather than always in the root. A disk can be organised without the host tool
touching it.

Everything below the surface works in terms of a directory and a name rather
than a path. sbfsWalkParent splits the last name off, walks the rest, and hands
back the two - and the separator stays on the end of the head, which is what
makes one rule cover every kind of path: "/x" leaves "/", which is the root;
"x" leaves nothing, which is where the machine already is; and "A/x" leaves
"A/", which is neither and needs no special case to say so.

Saving works in those two as well, and had to. The careful order a save uses -
make a temporary, write it, delete the original, rename the temporary - only
works if the temporary is made in the SAME directory as the file, because the
rename at the end changes a name and does not move anything. Renaming to a path
naming a different directory is refused for that reason, rather than quietly
being a lie the disk goes along with.

Three things this cost, all found by running it:

mkdir Apps/Deep made /Apps/Apps. The leaf was worked out into SbfsWanted and
then the head was walked - and walking goes through sbfsPathNext, which puts
every name it meets into SbfsWanted on the way past. The head's last name
landed exactly where the leaf was. It has somewhere of its own now.

rmdir took a directory with something still in it, which is the one failure the
whole design is arranged to prevent. Looking for children clobbered DP2 and
rebuilt it from the buffer and the entry count with the subtraction the wrong
way round, so the pointer walked off the end of the block and found nothing.
The comparison goes through a CALL now, like the two beside it, and DP2 comes
back on the entry because a RET puts it there. SplitDisk's "in use but not
reachable from the root" line is what caught it.

Refusing a name longer than twenty two used to read the twenty third character
of a shorter one, which is somebody else's string. It is measured now.

Tests/agree.sh is new and is the gate this rung was for: the same disk built
twice, once with SplitDisk and once with CosmOS, compared byte for byte. The
two share no code and only a written specification, and every field one writes
and the other only reads is checked there and nowhere else - which entry a
thing lands in, which block, what a directory's unused fields hold, the
version, the free count. It caught a wrong parent immediately when that was
broken on purpose.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E2JrLzFvuFX9fgi1LDRjrW

Tests/makedisks.sh

@@ -219,6 +219,12 @@ awk 'BEGIN { for (i = 0; i < 30; i++) printf "line %02d: ABCDEFGHIJKLMNOPQRSTUVW
 printf 'this is not a program' > rooted.txt
 "$TOOL" put "$DISKS/tree.img" rooted.txt >/dev/null
 
+# A blank disk for the machine to build a tree on itself. It starts as a version ONE disk
+# with nothing at all on it, because half of what this checks is that making the first
+# directory raises the version - the number says what is on a disk rather than what made
+# it, so a disk with no directories is flat whoever formatted it.
+"$TOOL" format "$DISKS/build.img" 128 2 >/dev/null
+
 # A disk for moving about on. Two directories hold a file of THE SAME NAME with different
 # text in it, which is the fixture the working directory needs: "notes.txt" has to mean a
 # different file from each of them, and the only way to see that it does is for the two to