Codify preview-first development and claim-specific verification

[kentwelcome]

Summary

• Add preview-first guidance for meaningful report hierarchy/navigation changes, with small copy fixes exempt.
• Match verification claims to structural checks, synthetic rendering, raw-trial audits, approved live extraction, or human evaluation; state remaining gaps.
• Add the repository-local preview-behavior-diff-summary skill using existing fixtures, ingestion, and rendering, with identical evidence across variants, explicit authored-content labels, loopback serving, browser checks, and temporary cleanup.
• Record tool lessons about external CLI paths, stale Python imports, and screenshot fallback.

Tracking: DRC-4803.

No production code, new renderer/framework, additional review gate, live model calls, or changes to evaluation/release protocols. No private artifacts included.

Verification

• git diff --check passed.
• Documented python3 tests/report-demo.py --serve --port 0 started, served the synthetic gallery, and exited cleanly on Ctrl+C.
• A throwaway fresh-process smoke selected the timing fixture, built it through build_reports, copied two variants, changed only authored summary text, and ran shipped ingestion/rendering. Trace bytes and decision chains remained identical; the updated narrative appeared in HTML. Temporary files were removed.
• This verifies the documented recipe, not live model compliance or reader comprehension.

Independent read-only review

• Standards: APPROVE; no material findings, decisions required, or optional suggestions.
• Specification and recipe correctness: APPROVE; no material findings, decisions required, or optional suggestions.
• Both reviewers followed REVIEWER_GUIDELINES.md, inspected all three files and supporting contracts, and made no changes.