Scroll the screen sideways, and by less than a cell

The screen could move one way, a cell at a time. Three registers were
missing and this adds them: a column origin so the map can be wider than
the screen as well as taller, and a pixel remainder for each axis so the
step can be one pixel rather than eight.

  0x36  Scroll column, in cells, wrapping at 128
  0x37  Fine X, 0 to 7 pixels
  0x38  Fine Y, 0 to 7 pixels

FINE DOES NOT CARRY INTO COARSE. Writing 8 to a fine register writes 0,
because only its low three bits mean anything. The alternative was for a
write of 8 to step the coarse register, and it was rejected for one reason:
a program that scrolls has to know where it has got to, and if the hardware
carries then the only way to find out is to read the register back. Keeping
them apart means the program already knows, because it did the arithmetic
itself. It is also what the machines this one is pretending to be did.

The renderer now draws one more row and one more column than fit and clips
them, because with a fine offset the screen no longer begins on a cell
boundary and the cells at two edges are partly off it.

videoPutCell follows the column origin as it has always followed the row -
a caller means a cell of the SCREEN, and the screen is a window onto the
map. The fine offsets are deliberately not applied there: they move the
finished picture by less than a cell, and there is no such thing as less
than a cell to write into. So a program may scroll to any pixel without the
console's idea of where row three, column five is moving underneath it.

Grid now scrolls diagonally, a pixel a frame, in four port writes and two
carries. It moved eight pixels every fourth frame before, which reads as
the picture jumping rather than travelling.

Seven checks, each one the same program with one register changed, so what
is compared is where the picture stopped. Breaking fine X, fine Y, the
column origin, the three-bit mask, or the console's use of the origin each
fails exactly one of them.

Grid's own two checks had to be rewritten, and the reason is worth keeping:
they asked whether pixel 4 was a grid line, which was really a check that
the scroll happened to be at a cell boundary. A picture that moves a pixel
a frame can only be asked things that are true at every offset - that it
repeats every eight pixels, and that one band of eight rows holds different
colours from the next.

Also repairs docs.sh, which found the minimal CosmOS application by taking
the first asm block in the README. Documenting a program with an example
above it made that a different block, and the check complained that the
minimal application had no #Base about something that never claimed to be
one. It looks under System Services now.

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

Tests/docs.sh

@@ -510,7 +510,15 @@ else:
     # The minimal application in the same section is what somebody copies, so it is the
     # part of the map most worth being right. It went stale across the doubling while the
     # table above it was corrected.
-    example = re.search(r"```asm\n(.*?)```", readme, re.S)
+    # ---- Found by its section, not by being first ----
+    #
+    # This took the first asm block in the file, which was the minimal application right up
+    # until somebody documented a program with an assembly example above it - and then this
+    # said the minimal application had no #Base, about a block that was never claiming to be
+    # one. The example lives under System Services; that is what identifies it.
+    services = readme.split("### System Services:", 1)
+    example = (re.search(r"```asm\n(.*?)```", services[1], re.S)
+               if len(services) > 1 else None)
     if not example:
         problems.append("the CosmOS README no longer shows a minimal application")
     else: