docs: define clean-room frontier merge-queue PRD

[rbtr]

Summary

• Add a Shunt-specific product and behavior contract for the frontier merge-queue model.
• Define safety, ordering, progress, durable-state, Bolt, and recovery requirements.
• Establish acceptance criteria and a clean implementation sequence.

Scope

This is a docs-only design PR. It intentionally does not promote an existing implementation into the product contract. Follow-up implementation PRs should be rebuilt from docs/frontier-prd.md and reviewed against its acceptance criteria.

Follow-up work

• Implement the neutral domain/checkpoint model with Bolt coverage.
• Implement deterministic serial frontier reduction.
• Add recovery, invalidation, ordering, and failure-isolation tests.
• Evaluate fan-out and Postgres separately after the serial behavior is measured.

Validation

• git diff --check
• Public-copy and sensitive-reference scrub