docs: design borrow-safe sum matching and guard review loops

[sanohiro]

Summary

• specify borrow-safe, non-consuming matching for admitted Option, Result, and user-sum payloads
• bind checked HIR, ownership cleanup, MIR lowering, interface, and acceptance-test responsibilities
• close review findings around exact borrow provenance, projection-only cleanup, nested matches, clone support, and Option/Result ownership wording
• prevent repeated descendant full-diff reviews unless an authoritative closure matrix is explicitly reopened

Verification

• scripts/test-pr-workflow.sh
• scripts/pre-pr.sh --base origin/main --reviewer codex --review-log <log> --findings-fixed --owner-test pr-workflow -- scripts/test-pr-workflow.sh

Implementation of borrowed sum projection remains deferred; this PR establishes its reviewed contract and process guard.