fix: a picture prints as the picture, where and as large as the teacher asks

The black bars above and below a printed picture were neither in the
picture nor in the layout. A PNG or JPEG in a tab of its own is shown by
the engine on a viewer page whose background is rgb(14, 14, 14), and Qt
prints page backgrounds by default; 0.2.6.1 printed that page.

A picture is now taken as itself — drawn onto white, capped at 300 dpi
across Letter — and laid out on the printer's paper by the preview, with
Size (as large as fits down to 25%), Position (nine squares), Paper
(automatic, upright, sideways) and Fill the whole page, which says it
cuts the edges off and is never the default. The preview is the PDF that
prints, laid out again after each change. Worksheet PDFs and lesson pages
get no picture controls and are unchanged.

Page backgrounds are no longer printed for lesson pages either, as in
Chrome; Blink darkens light text when it does that, so a white-on-black
page prints readable grey on white. "Colour" is "Color" in the print
interface and the manual.

Lab: 135 print checks, now including eight picture layouts measured on
the job file CUPS kept — byte-identical to the preview, within 1 point of
where asked, no ink anywhere else. Printed from the real session with the
pointer: 50% top left, Elmer-66, the same. Not yet on paper.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/wonder-ink.md

@@ -244,6 +244,39 @@ printed. Print pressed again while a preview is open does nothing, and so
 does a page calling `window.print()` meanwhile. The PDF viewer's own print
 button opens the same preview.
 
+### Pictures (0.2.6.2)
+
+A picture a lesson opens in a tab of its own — a colouring page as a PNG
+or JPEG — printed in 0.2.6.1 with **black bars** above and below it.
+Measured: the bars were neither in the picture nor in the page layout.
+The engine shows a lone picture on a viewer page whose background is
+rgb(14, 14, 14), scaled to the window, and Qt prints page backgrounds by
+default; the dark page went onto the paper around the artwork.
+
+So a picture is no longer printed as that page. The preview takes the
+picture itself — drawn onto white, which is what transparent parts are on
+paper, at up to 3300 pixels (300 dpi across Letter) — and lays it out on
+the printer's own paper, and the teacher chooses:
+
+* **Size** — as large as fits, 75%, 50%, 33% or 25% of that;
+* **Position** — nine squares, like the page itself;
+* **Paper** — automatic (landscape for a wide picture), upright or
+  sideways;
+* **Fill the whole page** — covers the printable area and says it cuts the
+  edges off. Nothing is cropped unless this is ticked.
+
+The preview is the PDF that will print, laid out again after every change.
+`tools/lab/wonderink-print-check` measures it on the file CUPS actually
+kept (`PreserveJobFiles`), which is byte-for-byte the preview's: the
+picture within 1 point of where it was asked for, the paper turned as
+asked, and no ink anywhere else on the page. Worksheet PDFs and lesson
+pages have no picture controls; nothing about them changed.
+
+Page backgrounds are also no longer printed for lesson pages, as in
+Chrome, where "Background graphics" is off unless asked for. A white-on-
+black page prints dark grey on white: Blink darkens light text when the
+background is left off, measured.
+
 Temporary PDFs live in the teacher's `$XDG_RUNTIME_DIR` and are removed
 when the preview closes.