Reading a script must not move the person who started it

A script is fetched a block at a time while its lines run, and its name
is resolved afresh for every block. A name with a drive in front of it
moves the machine to that drive on the way past - sbfsWalk calls sbfsUse
- so a script found in the system's place on drive 0, started by somebody
standing on a disk of their own, ran its lines on the system disk.

Always possible with "do 0:/Apps/setup.sh", and reachable by typing a
name now that the search finds scripts the same three places it finds
programs. The drive is kept across each fetch and put back after it, at
both places a script's name is resolved.

The test has to work for it. A script that fits in one block is read
entirely while it is being opened, and the opening was never the hard
part; and the keep in scriptFill cannot be broken on its own, because
scriptOpen has already written the variable down. So the script on the
disk crosses two block boundaries and moves itself between them: what it
says about where it is standing is 1 before the move and 0 after.

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

Tests/input/cosmosScriptDrive.in

@@ -0,0 +1,5 @@
+drive 1
+drive
+where.sh
+drive
+exit