engine: resolve bisection from durable ordered evidence

[rbtr]

Summary

• Persist the complete ordered frontier state: lineage, base anchors, exact-key evidence cache, and held leaves survive restarts through the Bolt checkpoint.
• Build every root child on its root's fixed base commit and scope cached gate results to that anchor, the merge style, and the ordered PR heads.
• Reject checkpoint evidence that no longer matches the forge: a changed head, a closed or ineligible PR, or a staging branch outside the configured namespace discards in-flight state and re-derives fresh work.
• Delete only staging branches matching the configured staging prefix, so a malformed checkpoint cannot target another branch.
• Keep Postgres as a supported shared store for replica leases; bbolt is the recommended local durable store.

Stacked on #76.

Validation

• make lint test build
• executable frontier model plus checkpoint, restart, invalidation, and ordering tests
• independent review of the final diff