A pull's looks show the before and after pairs on a real pull, and say why when main has no picture of a page #9733

For: the box #9669 left open. 1bff6d022 pairs main's picture with the head's in src/web/pages/looks.ts, and test/pull-looks-pair.test.ts holds the pairing in a fixture. Nobody has yet seen the pairs on a real pull, or decided what a page with no picture on main shows.

Done looks like:

• [ ] On a pull that touches src/web/ and was opened after 1bff6d022, its looks show each page as a pair, main beside head, at 1440 and 390. The pull for this issue links one such pull as the proof.
• [x] A page with no picture on main (a new page, or main's looks not taken yet) shows the head's picture alone, with one line saying main has no picture of it. It never shows a broken image or an empty frame. A test holds this.
• [x] bun scripts/design-lint.ts passes, and so do test/looks.test.ts and test/pull-looks-pair.test.ts.

Files: src/web/pages/looks.ts and test/pull-looks-pair.test.ts. Risks: main's pictures are kept apart from the pull's. Read them only through the looks actions the page already uses, and never from a path built out of user text.

Reviewed pull #9737 at its head: ticked A page with no picture on main (a new page, or main's looks not taken yet) shows the head's picture alone, with one line saying main has no picture of it. It never shows a broken image or an empty frame. A test holds this.; bun scripts/design-lint.ts passes, and so do test/looks.test.ts and test/pull-looks-pair.test.ts.. left open: On a pull that touches src/web/ and was opened after 1bff6d022, its looks show each page as a pair, main beside head, at 1440 and 390. The pull for this issue links one such pull as the proof. — No real pull is linked as proof; the builder said it could not see pulls looks..

Left open after pull #9737 landed: 1 unticked item remains (On a pull that touches src/web/ and was opened after 1bff6d022, its looks show each page as a pair, main beside head, at 1440 and 390. The pull for this issue links one such pull as the proof.). The planner files each as its own issue and closes this one; a person may close it, tick what is done, or say what is left. Until then the crew does not pick it again.

Closed by the planner: the shipped work (pairing in 1bff6d022, the no-picture line in #9737) is on main. The open box is a look at a real pull, which needs a browser the crew's sandboxes do not have; every pull touching src/web/ now carries the pairs, so a person sees it on the next one. Not re-filed as builder work.

Correction to the comment above: not closed, deferred behind #594. A person's ask stands in this issue, so the planner does not drop it, and its open boxes need a browser the crew's sandboxes lack. A person ticks them by eye (or runs the tool where Chromium is present) and closes it.

Correction: not deferred either. #594 already waits on this pillar chain, so the dependency would loop. It stays open, held for a person: its open boxes need a browser the crew's sandboxes lack, so no builder is handed it. A person ticks them by eye and closes it.