Flip says the true thing, and the checks that let it lie

Two bugs, both in what the demo claimed rather than in the device.

The assembler has no string escapes, so the "\n" written in a literal
printed as a backslash and an n. A newline is a byte; Say.asm has always
written one as 0x0A 0x00 and this now writes it out of the console port.

And the line whose whole job was to still be there afterwards was wiped
out on the way back, because the program called osTakeScreen - which
restores the screen AS IT WAS BEFORE, so the tidy-up erased the one thing
the demo was pointing at. It did not need saving: nothing it touches is
the shell's. A program that damages nothing should not ask, and asking
anyway costs it the screen it was standing on.

Which turned out to be untrue as written, and that is the third thing.
Flip drew with a tile of its own, and the system copies the font back
over every tile at exit - so the filled screen went blank the moment the
program left, and the check that the system put the display back could
not tell a restored screen from an abandoned one. It passed with the
restore deleted. So did the check that a program can show the other
screen at all: a blank screen counts as one colour just as well as a
filled one does.

Now it fills with 0x0A, which is an asterisk in one of the reversed
colour schemes: paper is the colour and ink is black, so a whole screen
is drawn with NO TILE REDEFINED and it survives leaving. Both checks ask
for the commonest colour in the picture rather than counting colours or
naming a pixel - the font's only blank glyph is the space, whose
attribute nibble is nought, so a filled screen is always a pattern and
which pixel lands on paper depends on the character.

Both were re-broken afterwards and both failed this time.

Also cosmosFlip, a transcript test, which is what would have caught the
printed backslash in the first place.

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

Programs/CosmOS/README.md

@@ -725,7 +725,7 @@ from every assembly file in it. Several are old programs written for the bare ma
 | Files | Writes a file, reads it back, renames it and deletes it, in 675 bytes, including nothing but the service names. It is what says a program does not need a filesystem inside it. |
 | Break | Stops itself twice with SWI osBreak, so that the registers can be seen changing between one stop and the next. |
 | Grid | The first program to use the screen as a screen. Redefines a tile above the font, fills all 128 map rows, and scrolls it diagonally a pixel at a time. |
-| Flip | Draws a whole screen into the bank that is not being shown, waits, and then shows it in one byte out of one port. It deliberately does not put the displayed screen back, because that is the system's to restore - a program that faulted while flipped could not have. |
+| Flip | Draws a whole screen into the bank that is not being shown, waits, and then shows it in one byte out of one port. It writes nothing else at all - not a tile, not a colour - so it does not ask for the screen to be saved, and the line it printed is still there when it comes back. It deliberately does not put the displayed screen back either, because that is the system's to restore: a program that faulted while flipped could not have. |
 | Edit | A line editor. |
 | Stream | Reads an 84,000 byte file through a buffer of 256, which is what says a file bigger than Data Memory can be read at all. |
 | Type | Prints a named text file a block at a time, including one too large to fit in Data Memory. |