A preview's toolbar page says what its pull changed and whether its check is green, each link a 44 px tap on touch screens #4205

Replaces #4088, whose pull #4100 met every review but one and then sat. If the branch scout/4088-a-preview-s-toolbar-page-says-what-its-p is still on the forge, start from its commits; the reviews accepted its diffTotals(), runStateBadge, .change-line and the stat cached by head oid.

Done:

• [ ] On toolbarPage (src/web/studio/toolbar.ts) for a preview, under the head line: 'N files, +A −R' (the diff summary's own .add/.del totals) linking to the pull's Files page, and the latest check as §5's badge (passed, failed, running, cancelled, none) linking to its log.
• [ ] The diff stat is read once per head oid, so the toolbar page stays under 50 ms in app in the test.
• [ ] The line's links are at least 36 px tall, and a @media (pointer: coarse) rule makes them 44 px (docs/DESIGN.md, Density); the class is named in §5d with that note.
• [ ] Production's toolbar page is unchanged; a viewer who may not see the pull gets a 404.
• [ ] Tests for a pull with a green check, a red one and none.

Files: src/web/studio/toolbar.ts, src/web/diff-view.ts, src/web/style/content.ts, docs/DESIGN.md, a test. Before pushing: bun x tsc --noEmit, bun test test/design-lint.test.ts and the toolbar tests.

Parked by the crew: the check failed ("fork at the origin, and propose a change through it > Fork lands on the job, then on the fork's mirror saying whose it is") and the decider saw no point in another try (pull #4219 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.