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>

docs/images/README.md

@@ -0,0 +1,21 @@
+# Generated figures — do not edit
+
+**Every PNG in this directory is output, not an asset.** Each one is produced by running
+the BASIC listing shown immediately above it in the chapter, through
+`tools/screenshot.c`, and it is checked in only because a reader on the forge has no
+build tree to generate it from.
+
+To change a picture, **change the listing** and regenerate:
+
+```sh
+cmake --build build-akgl --target docs_screenshots
+```
+
+Editing a PNG here by hand puts the picture out of step with the code it illustrates,
+which is the exact failure this arrangement exists to prevent — and the `docs_screenshots`
+test will fail on it, because it re-renders every figure and compares byte for byte.
+
+The tag that ties a figure to its listing is `screenshot=NAME` in the fence info string,
+and `MAINTENANCE.md` documents it along with `size=WxH` for a figure that needs a surface
+other than 320 by 200. A tagged block with no image here fails `docs_examples` in both
+build configurations, so a figure cannot be added to a chapter and then forgotten.