engine: stage optimistic bisection prefixes

[rbtr]

Summary

• Stage speculative bisection nodes against the likely prefix when fan-out exceeds one.
• Promote a speculative result only when its exact evidence key still matches the resolved frontier; otherwise supersede and re-stage.
• Document the speculative start/promote/supersede metrics and the durable-store guidance.

Stacked on #77.

Validation

• make lint test build
• speculative ordering, supersession, and metrics tests
• independent review of the final diff