CC-519: Add structured reviewer coverage and finding contracts

[screenleon]

What changed

• add the gate_reviewer_result_v1 schema with a closed verdict vocabulary, complete coverage declarations, stable actionable findings, and scope-bound evidence references
• extend current scope manifests with an immutable reviewer reference index and enforce path, digest, and line-bound validation before synthesis
• make structured JSON verdicts the machine source of truth while preserving reviewer reports in pr_gate_result_v3
• align all reviewer prompts, runtime verification, fallback generation, and documentation entry points
• consolidate shared PR-gate fixtures and update affected-suite mappings and test documentation

Why

Reviewer completion was coupled to Markdown heading formatting, so duplicated or omitted prose could turn a substantively complete review into a protocol failure. Reviewer outputs also lacked one deterministic contract for coverage, evidence, findings, and aggregate verdict parity.

Impact

Selected reviewers now produce machine-verifiable, scope-bound results. Protocol defects fail closed as INCOMPLETE, while actual reviewer verdicts remain distinct and deterministic across sequential and parallel execution.

Validation

• affected-test PR gate: GO before the final comment-only Behavior/Steps annotation; the owner explicitly accepted retaining that GO without a gate rerun
• bash tests/bin/run-all-tests.sh: PASS — 97 passed, 0 failed, 0 skipped
• bash tests/bin/run-tests.sh --verify-full .pm-dispatch/test-results/latest-full.json: verified authoritative full PASS for the committed tree
• git diff --check: PASS