A preview's toolbar page says what its pull changed and whether its check is green, each a link

P6's next increment: the pill's page in a preview says what you are looking at the change of.

Done:

[ ] On toolbarPage (src/web/studio/toolbar.ts) for a preview, under the head line: 'N files, +A -R' linking to the pull's Files page, and the pull's latest check verdict (passed, failed, running, none) linking to its log, drawn with §5's badge.
[ ] Production's toolbar page is unchanged, and a viewer who may not see the pull still gets a 404.
[ ] The reads are timed as Server-Timing phases; the page stays under 50 ms in app in the test.
[ ] Tests for a pull with a green check, a red one and none; bun x tsc --noEmit passes.

Files: src/web/studio/toolbar.ts, a test. Risk: other toolbar issues edit the same function; rebase onto whichever lands first.

A preview's toolbar page says what its pull changed and whether its check is green, each a link #4088