Say a temporary is one in the entry, not in its name

Saving something that already exists writes a temporary, deletes the
original and gives the temporary its name, so that nothing is lost if the
writing fails. The temporary was told apart from a real file by being
called sbfs.part or sbfs.out - and those are legal names. Starting a save
deleted whatever answered to one as stale scratch, so saving anything at
all in a directory destroyed your own file of that name there, silently.

Flag bit 0x04 now says it. The property is not in the contents - the same
bytes become the finished file the instant the rename lands - so it belongs
in the entry, which is the thing the commit changes. sbfsCreateTempAt is
the door temporaries come in by, the commit writes the flags flat along
with the name, and cleanup wipes what it finds only if the entry says it is
ours. Anything else stops the save instead.

The bit is also the recovery. Both listings show an unfinished write rather
than sizing it, because the size in the entry is the room that was asked
for and not what was written: "<unfinished>" from dir, and a line from
SplitDisk saying the blocks are held and a rename brings the data back.
That was the gap in what the last commit documented - the data survived a
crash and nothing would show you where it was.

Four new agreement checks, three of which fail with the guards removed. The
fourth needed rebuilding first: both tests started on one disk, and the
first save ate the sbfs.part that was the second test's SOURCE, so the copy
failed for want of a file, never opened a stream, and passed while
reporting on nothing. A disk each. The fifth check forges the wreckage by
setting the flag on a finished file, since nothing here can crash a save
half way through.

No version bump: a committed file never carries the bit, so a disk this
writes is byte for byte the disk the old code wrote, which the whole-image
comparisons confirm. Only the wreckage differs, and older code reads that
as an ordinary file - which is what it did before.

Source/DiskTool/sbfs.h

@@ -97,6 +97,22 @@
 #define SBFS_FLAG_IN_USE    0x01
 #define SBFS_FLAG_DIRECTORY 0x02
 
+// A FILE BEING WRITTEN, WHICH IS NOT YET A FILE. Saving something that already exists is
+// done by writing a temporary, deleting the original and giving the temporary its name,
+// so that nothing is lost if the writing fails. The temporary has to be an ordinary entry
+// while that happens - it holds real blocks and needs a name - and the only thing that
+// distinguishes it from a finished file is that nobody has committed it yet.
+//
+// That is not a property of its contents. The same bytes become the real file the moment
+// the rename lands, so there is nothing to put inside it that would be true; it belongs
+// in the entry, which is the thing the commit changes. It was formerly told apart by
+// being called "sbfs.part" or "sbfs.out", and those are legal names a user may also
+// choose, so cleaning up by name could delete somebody's file.
+//
+// Cleared as part of committing. An entry still carrying it is the wreckage of a write
+// that stopped, and its blocks are spoken for until something clears it up.
+#define SBFS_FLAG_TEMPORARY 0x04
+
 // The root is not an entry. It is the absence of a parent, written as zero, which is why
 // the field is an index plus one and why a freshly zeroed entry is already in the root.
 #define SBFS_PARENT_ROOT 0