Name a drive in a path

"1:/notes", or "1:" on its own for wherever that drive already was. Done in
sbfsWalk, which is where every path in the system arrives - eight callers
between the shell, the config reader and the filesystem - so it works for
anything that takes a path rather than for whichever commands somebody
remembered to change.

NAMING A DRIVE GOES THERE AND STAYS THERE. Switching for the length of one
command and switching back reads better and cannot work: a path resolves to
a start block and a length, and those mean nothing without the drive they
were read from. A load that resolved on drive 1 and then read its blocks on
drive 0 would read the right blocks of the wrong disk.

A name beginning with a digit is still a name. The colon is the whole of
what tells them apart, and /2things is on the fixture disk to keep it that
way.

Two bugs, and the second is the interesting one.

SUB sets carry on a BORROW, so a character below '0' leaves it set - and
the test for "not a digit" branched on clear. Every prefix was ignored.

Then the leading-separator test reads the first character through DP0,
which sbfsPathDrive could not move because RET puts DP0 back the way it
found it. It advanced SbfsPathAt and DP0 still pointed at the digit, so
every prefixed path was judged relative and walked from the named drive's
working directory. IT ONLY SHOWED WHEN THAT DRIVE WAS STANDING SOMEWHERE
OTHER THAN ITS ROOT, because a relative walk from the root is an absolute
one - so "cd 1:/2things" worked from a fresh boot and failed after "cd
1:/notes". The test does it in that order for that reason.

Copying between two disks is still not one command: each path resolves on
its own drive and the drive stays where the last path left it. That wants
Copy to change drives between blocks.

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

Tests/input/cosmosDrivePath.in

@@ -0,0 +1,11 @@
+cd 1:/notes
+drive
+cd 0:/
+drive
+cd 1:/2things
+drive
+cd /
+cd 2things
+drive
+cd 9:/
+exit