upstream follow-up (2293): perf(graph): reuse indexed shortest-path workspaces #629

Follow-up from the review of recent upstream Kpa-clawbot/CoreScope PRs: 2293. Read the upstream PRs and their linked issues for details (read only; never comment there).

Upstream

Upstream (merged): bridge and coverage scoring convert keys once and reuse indexed distance/predecessor workspaces, with at most one heap item per vertex; same algorithms, normalisation and publication. Fixes upstream issue 2226.

What to do here

We reduced graph-score warmup allocations in #587 (#561). Measure our bridge/coverage scoring on a 2,000-node / 10,000-edge graph and compare with the upstream approach; port only if it is a measurable win on top of #587 (benchmark before/after, allocs and time).

General rules: audit before porting, red-before/green-after tests with a killed mutant for every behaviour change, no SQL under PacketStore.mu, cmd/server stays read-only, no new map[string]interface (Kpa-clawbot#1383), CSS variables only.