Grid took the disk's bank number, and gave the screen back untidy

Found by playing with it: after running Grid, the shell could not start
anything by name and dir said the disk was empty. Several commands after
the program that did it had exited, and nothing had said a word.

BANK NUMBERS ARE ONE NAMESPACE FOR THE WHOLE MACHINE. Grid registered video
memory as bank 3, which is the number CosmOS gives the disk's buffer when
it mounts - and that does not fail, it succeeds. Every read the filesystem
made afterwards came out of video memory. Grid uses 4 now, and the CosmOS
README has a table of who owns what, because the one place this was written
down was a line in a service description about sbfsMount.

Nothing hands bank numbers out and nothing refuses one that is taken. If
programs start wanting banks routinely, a service that allocates them is
what should exist rather than a longer table - noted there rather than
built, since one program wanting one bank is not yet a system.

Also puts the cursor home on the way out. The map was emptied and the
console was not told, so the shell carried on writing from wherever the
cursor had been standing when Grid started - twelve rows down a screen with
nothing on it. Clearing is what homes a cursor and it costs one write.

The regression test runs a program by name, then Grid, then the same
program again; the second one is the check. Putting Grid back on bank 3
fails it.

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

Tests/expected/cosmosSlowDisk.out

@@ -6,7 +6,7 @@ Snake.sbx               2164
 Keys.sbx                664
 Say.sbx                 156
 Break.sbx               149
-Grid.sbx                539
+Grid.sbx                543
 notes.txt               21
 hi.script               121
 bad.script              45