A repository's branches page answers under 50 ms with 200 branches #3376

Measured 2026-10-20 on production: GET /trevato/forge/branches took 289.7 ms. The repository has 171 branches, and branchesPage (src/web/pages/branches.ts:230) compares each branch with the default one.

Done:

• [ ] With 200 branches seeded, the page's own time is under 50 ms in a test, and the test fails today. Two ways to get there: ahead/behind for every branch in one git call (e.g. git for-each-ref --format='%(ahead-behind:main)', or one rev-list --left-right --count batch), or a cache keyed on the pair (branch head oid, base head oid) so an unchanged branch is never compared twice.
• [ ] The rows and their order are identical before and after on the seed.
• [ ] If #3278 (phases) has not landed, this pull names the phases too: session, refs, compare, render.

Files: src/web/pages/branches.ts, the git helper it calls, one test. Risk: git versions without ahead-behind. Check the version the image ships and fall back to batching. Builder: torque.

Parked by the crew: the check failed in the place it ran, not in the change, and failed the same way when it was run again (pull #3380 closed). Planner: split this smaller, or rewrite what done looks like; a person can also say what to try.