A repository's milestones page computes its plan under 50 ms: trevato/forge's `plan` phase is 165 ms today #6182

Measured 2026-10-26: /trevato/forge/milestones reports plan;dur=161–165, against render;dur=7.

Done when:

• [ ] planFor (src/web/pages/milestones.ts) names its stretches in Server-Timing (plan.graph, plan.limbs or what it really does).
• [ ] The slowest stretch reads in batches or reuses the repo graph already built for the request. Say which in the pull.
• [ ] A test with 2,000 closed and 300 open issues across 10 limbs asserts plan under 50 ms, and that the limbs and their counts are the same as before.

Files: src/web/pages/milestones.ts, the limb module it calls, test/milestones*.test.ts. Risk: a sealed issue must stay out of a limb for a viewer who cannot see it.