Stop the prompt writing off the front of its own buffer

The prompt is the working directory's path, worked out each time by walking
the chain of parents up to the root. The names arrive deepest first, so
they are written backwards from the end of a 127 byte buffer - and nothing
bounded that walk.

Nothing bounds the depth either. A path given to one operation is capped at
95 characters and a 22 character name, but "mkdir a" and "cd a" are each
far inside that and can be repeated forever. Six directories of 22
characters is 132 characters of path, and at that point the walk wrote down
past the front of CwdText and into what the assembler had laid out below
it: the shell's own command names. ExitName sits five bytes under, so the
word "exit" went first and the shell stopped recognising the command for
leaving. Measured, not deduced: fine at five levels, gone at six.

The walk now counts the room it has left, byte by byte, and stops. What is
already written is the DEEP end of the path, which is the end worth
showing, so it is cut at the front and three dots say so - out of three
bytes held back from the count, so there is always somewhere to put them.
Twenty levels deep the prompt shows the last five and every command still
works.

cosmosDeep records that, and records it by running help, cd and exit from
down there rather than by looking at the prompt: a wrong prompt is
cosmetic, and this was writing into other variables. It fails with the
bound removed. The tree is built by SplitDisk because a path that long
cannot be given to mkdir in one piece - which is the same fact that makes
the depth unbounded.

The three path limits are written down in the README now, including which
one actually binds. The other two do not: the longest path on a full
install is 21 characters.

Tests/manifest

@@ -333,6 +333,18 @@ cosmosMore           | CosmOS/Source/cosmos.asm           | run      | cosmosMor
 # part-block and 84,000-byte files are copied through one buffer; Compare checks the copies
 # and a same-sized file whose only difference is deep into the input.
 cosmosCopyCompare    | CosmOS/Source/cosmos.asm           | run      | cosmosCopyCompare.in | -   | disks/copycompare.img
+
+# A working directory deeper than the prompt can print. The prompt is built BACKWARDS into
+# 127 bytes, and nothing bounds how deep the directories go - a path is capped at what one
+# operation can name, but "cd" a level at a time is well inside that and can be repeated.
+# So the walk wrote down past the front of its own buffer and into what the assembler put
+# below it, which was the shell's command names: at six directories of twenty two
+# characters the word "exit" was gone and the shell no longer knew how to stop.
+#
+# WHAT IS RECORDED IS THAT THE COMMANDS STILL WORK, and that is the whole point of running
+# help and cd and exit from down there rather than just looking at the prompt. A prompt
+# that is merely wrong is a cosmetic fault; this one was writing into other variables.
+cosmosDeep           | CosmOS/Source/cosmos.asm           | run      | cosmosDeep.in      | -     | disks/deep.img
 # Reading a disk that has directories on it. The machine can walk a path at this point but
 # cannot make a directory, so the disk is built by the host tool and read here - which is
 # the two implementations checking each other rather than either checking itself.