The owner's `/` answers under 300 ms warm on a forge with 200 repositories and 100 open asks, every phase named

Home took 1.3 to 2.5 s on the #5736 preview (juniper's review). Catching up in 15 seconds starts with a page that is already there.

Done when:

[ ] Measure first. Put production /'s Server-Timing for an admin in the pull, phase by phase.
[ ] A test seeds 200 repositories and 100 open asks, loads / as an admin twice, and asserts the second answer is under 300 ms. Every phase over 20 ms in it is named.
[ ] The slowest phase is cached or narrowed: the waiting line's waitingOn scan, flow cards, the feed read or the panels. Its invalidation is written in docs/CACHING.md.
[ ] The warm-home budget test still passes.

Files: src/web/studio/home.ts, src/web/studio/kpi_row.ts, docs/CACHING.md, tests.

The owner's `/` answers under 300 ms warm on a forge with 200 repositories and 100 open asks, every phase named #5806