fix: the figures carried a chunk WebKit refuses, which is why a page never loaded

The user's report was precise and I misread it twice: opening trigger.page as a *file* rendered the
page with no image, while opening it through the app's `help:` URI spun forever. Both are one cause.
The three figures carried a `bKGD` chunk -- and the PNG specification does not permit `bKGD` on an
image with an alpha channel, which all three are (colour type 6). ImageMagick both writes and reads
it, so my "decodes fully" check proved nothing: I tested with a decoder more permissive than the one
that has to load the file. WebKit rejects it, so the image never appears; over `help:` the fetch
never completes either, so the page's load event never fires and Yelp's spinner never stops.

It came from `import`, which records the root window's background. Every figure now carries only the
chunks a decoder is guaranteed to accept (IHDR/IDAT/IEND), and `tools/make-help-shots.sh` normalises
its output at the end so a re-shoot cannot reintroduce it -- with the reason written where the next
reader will be, since "ImageMagick reads it" is exactly the trap.

`tools/check_help.sh` gains the rule, because the four yelp-check subcommands and the structural
rules are all blind to a figure no decoder will accept: a `bKGD` on colour type 4 or 6 is a finding,
checked directly rather than through a permissive tool. It is a whitelist because a decoder is one.

Verified: the rule fires on the committed figures (colour type 6, bKGD present) and passes on the
fixed ones; the figures still decode and are still 1280x800 / 938x420; check_help.sh is green on the
source and installed sets; make -C build test-help passes.

tools/make-help-shots.sh

@@ -221,4 +221,16 @@ APP_ARGS="--plots 1" start_app "$tmp/math-editor.log" \
 sleep 10
 grab math-editor "$tmp/math-editor.log" "$boxes_math" pick_other_window
 
+# The last step, and it is not cosmetic: `import` records the root window's background, so the
+# PNG leaves ImageMagick with a `bKGD` chunk -- forbidden by the PNG specification on an image
+# with an alpha channel (colour types 4 and 6), which these are. ImageMagick writes it and reads
+# it back happily, so a round-trip through `magick` proves nothing; a stricter decoder refuses
+# the file. In Yelp that showed as a figure that never loaded, and over the `help:` URI as a page
+# that spun forever, because the load event never fired. Normalise every figure to the chunks a
+# decoder is guaranteed to accept, whatever the grab produced.
+for f in "$out"/*.png; do
+  magick "$f" -strip -define png:exclude-chunk=bKGD,cHRM,tIME,tEXt,tXMP,exif \
+    -define png:compression-level=9 "$f"
+done
+
 echo "make-help-shots: done"