A pull's pairs line counts pages, not pictures, when a page has both a phone and a desktop look #9820

Later: It hardens a line that already ships and works on real pulls; no pillar needs it before the merge-queue work.

From #9768's open box. The line N pages beside main · M new to this pull shipped (src/looks/pairs.ts, pairCounts and pairsLine), but test/pull-looks-pair.test.ts only seeds phone looks, so nothing pins how a page pictured at both 390 and 1440 is counted.

Done looks like:

• [ ] test/pull-looks-pair.test.ts seeds main and head looks of / at both phone and desktop, head looks of /about at desktop only, and main looks of /about at phone only, and asserts the line reads 1 page beside main · 1 new to this pull (a page counts as beside main only when main and head share a width).
• [ ] The test fails if pairCounts counted pictures instead of pages, or counted a page as beside main when main and head pictured it at different widths; if the code already behaves, the test pins it and the pull says so.
• [ ] bun test test/pull-looks-pair.test.ts test/looks.test.ts passes.

Files: test/pull-looks-pair.test.ts; src/looks/pairs.ts only if the test finds a fault. Risks: none beyond the test fixture; keep withLooksSwapped so other tests' rows are restored.