Generate the documentation's figures from the listings they illustrate

Chapters 6 and 8 described what a verb draws in prose. Eight figures now show
it, and each one is produced by running the BASIC listing printed immediately
above it -- so a picture cannot drift away from the code beside it, which is the
way a screenshot goes wrong and the way nothing notices.

tools/screenshot.c is a second SDL host, much smaller than the frontend: dummy
video driver, software renderer, run to completion, read the target back, write
a PNG. It draws no text layer on purpose, so a READY in the corner is not noise
in a figure about BOX and no font has to be resolved.

tools/docs_screenshots.sh reads the new screenshot=NAME fence tag straight out
of the markdown. size=WxH is the second tag, and SCALE's figure uses it: the
point being made is a 320x200 listing filling a larger window, which cannot be
made on a 320x200 surface.

Two gates, answering different questions. docs_examples fails a tagged block
with no image, in both configurations, so a figure cannot be added and
forgotten. docs_screenshots -- a CTest, AKGL build only -- re-renders every
figure and compares byte for byte, so a listing edited without regenerating
fails. Only the second catches a stale picture.

The PNGs are checked in because a reader on the forge has no build tree, and
docs/images/README.md says loudly that they are generated. Regenerating is never
part of a build: the target is run deliberately, so a make cannot put eight
binary diffs in front of whoever ran it.

Drawing the BOX figure caught a defect in TODO.md itself. Deviation 16 claimed
in bold that BOX fills on a negative angle while its own paragraph said the fill
was filed rather than implemented. BOX cannot fill, and filled_rect is reached
by no verb as a result.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitea/workflows/ci.yaml

@@ -214,6 +214,11 @@ jobs:
       # they are the ones that must keep passing when SDL is present, *and* they
       # are what proves the SDL frontend changes no output anywhere in the
       # corpus.
+      # docs_screenshots runs here and nowhere else, because rendering a figure
+      # needs the SDL half. It re-renders every picture in docs/ and compares it
+      # byte for byte with the checked-in copy, so a chapter whose listing was
+      # edited without regenerating fails on this push rather than shipping a
+      # picture of code that no longer exists.
       # akgl_typing reports Skipped here and that is correct: it needs a real X
       # server, a window manager and xdotool to type at a focused window, and a
       # CI runner has none of the three. It is a developer-machine gate, and the