engine: harden queue lifecycle recovery

[rbtr]

Summary

• Resume and reject active batches safely after restart, bound missing-gate and cancelled-auto-merge recovery, and discard batches that lose eligibility.
• Add the frontier PRD, rewritten in controlled technical English, as the behavior contract for the ordered-frontier work in engine: resolve bisection from durable ordered evidence #77.

Stack: #76 targets main. #77, #78, and #79 build on it.

Validation

• make lint test build on the code change
• STE structural lint: 0 hard violations
• independent review of the diff