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

Tests/manifest

@@ -968,6 +968,15 @@ cosmosStartupBad     | CosmOS/Source/cosmos.asm           | run      | cosmosSta
 # So this runs a program by name, then Grid, then the same program again. The second one is
 # the check. dir at the end says the disk is still there to be read.
 cosmosGrid           | CosmOS/Source/cosmos.asm           | run      | cosmosGrid.in      | 60000000 | disks/cosmos.img
+# ---- The screen nobody is looking at ----
+#
+# Flip draws a whole screen into the bank that is not shown. What is checked here is not the
+# picture - video.sh has that - but the LINE IT PRINTS, which is the thing the program claims
+# will still be there when it comes back. It was not, twice over: the assembler has no string
+# escapes, so a "\n" written in a literal printed as two characters; and the program called
+# osTakeScreen, which restores the screen as it was BEFORE the line was printed and so wiped
+# out the one thing it was pointing at. A program that damages nothing should not ask.
+cosmosFlip           | CosmOS/Source/cosmos.asm           | run      | cosmosFlip.in      | 60000000 | disks/cosmos.img
 # Which disk the registers mean. Several disks are one controller with a drive register
 # rather than several devices, because a port is an immediate byte inside the instruction
 # that names it and a program cannot compute one. Run with a single disk, so drive 1 is a