The front page's build shows its stretches in Server-Timing on a cache miss #6273

Follow-up to #6178 / pull #6264. / never calls shelfRows: its shelf is frontPage (src/actions/front.ts), cached, and #6264 only splits it into shelf (the read) and tiles (drawing). On a cache miss the 70–90 ms measured on / is spent inside buildFrontPage (picks, public apps, repos, owners, readability, runs), and none of it is named.

Done when:

• [ ] On a cache miss GET / sends front-* phases for the stretches of buildFrontPage (e.g. front-picks, front-records, front-repos, front-owners, front-rows), each once; on a hit it sends none of them.
• [ ] A test with two public apps asserts the names on a miss and their absence on a hit.

Files: src/actions/front.ts (take an optional lap callback rather than a Request, so actions stay free of http), src/web/studio/home.ts, test/shelf-laps.test.ts.

Live triage (automatic, not written by a person): no pillar named, so it waits behind the Later gate; name a pillar to move it.