Refuse a directory whose last entries cannot be named as a parent

A parent is an entry index PLUS ONE in two bytes, so entry 65535 has no
parent number: adding one wraps to zero, and zero is the root. Eight
entries to a block, so 8192 directory blocks reaches it and SplitDisk
formatted that happily.

It does not fail by refusing, which is why it was worth chasing rather than
reasoning about. Reproduced on a disk built for it: mkdir /deep/child, with
/deep at entry 65535, printed 'Made "/deep/child" as entry 0' and put child
in the ROOT. Listing /deep then showed nothing, because the search is for a
parent of 65536 and the entry carries zero - so the same mkdir succeeded
again, and again, and five entries called /child piled up in the root.
Duplicate names in one directory are the one thing rename refuses outright,
on the grounds that a search answers with whichever it meets first and the
rest can never be reached; this manufactured them one per attempt.

8191 blocks is the most, giving 65528 entries. Refused when formatting and
again when reading, in both implementations, because a disk claiming more
was made by something that never checked. On the machine only the high byte
of the count has to be looked at: anything from 0x20 up is too many.

Three checks, all of which fail with their guard removed. The machine's
disk claims the size rather than having it, so the test image is 64 blocks
that lie rather than sixteen megabytes that do not - mounting is refused at
the geometry, which is read out of block 0.

Tests/disk.sh

@@ -155,6 +155,23 @@ before=$(blocksFree tree.img)
 "$TOOL" mkdir tree.img /Empty >/dev/null 2>&1
 check   "a directory costs no blocks"     [ "$before" = "$(blocksFree tree.img)" ]
 
+# ---- A directory no bigger than the parent field can name ----
+#
+# Eight entries to a block and the parent is an index plus one in two bytes, so entry
+# 65535 has no parent number: adding one wraps to zero, and zero is the root. Such an
+# entry does not refuse what is put inside it. It writes the thing into the ROOT while
+# reporting the path that was asked for, and then cannot find it again - so the same
+# create succeeds over and over, piling up entries of one name in one directory, which is
+# the exact corruption rename exists to refuse.
+refuses "no directory past the wrap"      "$TOOL" format huge.img 65535 8192
+check   "the largest that fits"           "$TOOL" format huge.img 65535 8191
+
+# And a disk claiming one, which is what something that never checked would have written.
+# The claim is in the superblock, so it does not need a disk that size to be made.
+"$TOOL" format lying.img 64 2 >/dev/null
+printf '\x20\x00' | dd of=lying.img bs=1 seek=10 conv=notrunc status=none
+refuses "nor reading one that claims it"  "$TOOL" list lying.img
+
 echo
 if [ "$FAIL" -eq 0 ]; then
     echo "All $PASS disk tool checks passed."