Put CosmOS on the screen without changing a line of it

The console is now a display controller as well as a port: it owns a font, keeps a cursor,
handles newline, carriage return, backspace and wrapping, and scrolls. That is an ordinary
kind of chip - it is what a video terminal's character generator did - and it is the reason
this rung needed no changes to CosmOS at all. CosmOS already writes bytes to port 0x00.

It writes to BOTH the screen and standard output, which is deliberate. A machine with a
screen and a serial line is an ordinary machine, the emulator's standard output is that
serial line, and one console drives both. It is also what keeps all 165 recorded results
passing under Voyager, and what makes --screen work on the plain SplitBit: there is one
console and it drives everything it has.

Scrolling moves the video device's origin and no memory. The row arriving at the bottom is
cleared because the map is a ring and it holds what was there 128 rows ago; the rows going
off the top are not, and that is a hundred rows of scrollback nothing had to keep. The test
reads the register back rather than looking at the screen, because a console blitting rows
instead would look identical and cost twelve percent of a frame for every line printed.

The font is vendored from Hatchet-GPU with a note saying where it came from, since that
repository is not part of this one. 135 glyphs in ASCII order, which is the thing that makes
it worth keeping - PETSCII's whole inconvenience was that its order was not ASCII's, so a
machine using it needed a translation table in front of every string. Here the machine
subtracts 32. It is stored one bit a pixel and expanded into tile memory at reset: 1,088
bytes against 16 kilobytes.

Voyager gets a keyboard. A window has no standard input, and a machine blocking on it inside
a frame would stop drawing and stop answering, so a front end with a window installs a hook
that the console calls while it has nothing: it keeps the window alive and hands back a key.
The hook has to tell "nobody has typed yet", which happens sixty times a second, apart from
"the window has gone", which is the end of input - one value for both would have made the
first keystroke look like a closed machine. In line mode the console echoes what it is
given, because there is no terminal behind a window to do it and that was always the
terminal's job.

Tests/video.sh grew from 14 checks to 26, half of them about the console rather than the
device: those programs ask the video device for nothing and write bytes to port 0x00 like
every SplitBit program always has. Verified by breaking two things - removing the scroll
failed exactly the two checks about scrolling, and removing the cursor advance failed
exactly the three that depend on it.

Two video checks had quietly depended on palette entry 0 being black, which stopped being
true the moment a machine woke up able to show text. They now set what they are about to
look at, and a new check pins the waking state itself.

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

SplitBit Test Manual.md

@@ -158,6 +158,14 @@ the palette is what colours it, that the attribute nibble adds sixteen, that scr
 which row is on top, that the map wraps, and that an impossible mode is refused without
 stopping the machine.
 
+**Half of it is about the console rather than the device.** Those programs ask the video
+device for nothing at all: they write bytes to port 0x00, the way every SplitBit program
+always has, and the picture is what is checked. A character lands at the cursor and the
+cursor moves along, a newline starts the next row, backspace rubs out, the line wraps at the
+last column, and the screen scrolls by moving the origin rather than by moving 1,920 bytes -
+which is checked by reading the register back, because a console that blitted rows instead
+would look identical on the screen and cost twelve percent of a frame a line.
+
 Breaking the additive nibble fails exactly one check. Breaking the scroll origin fails
 exactly two. Moving every cell one pixel sideways fails the four about placement. That is
 what a screen test is supposed to do.