Expose visual explanation controls

[chubes4]

Summary

• expose visual explanation element/candidate limits and repeatable selectors in the public wordpress.visual-compare command contract
• normalize camelCase and kebab-case fields consistently for pair and matrix runs
• preserve bounded defaults while strictly rejecting malformed limits
• record effective options and selector inheritance in deterministic screenshot-backed evidence

Closes #1870

How to test

1. Check out this branch in a fresh clone.
2. Run npm ci.
3. Run npm run test:browser-visual-compare-contract.
4. Run npm run test:browser-visual-compare-fair-ratio.
5. Run npm run test:browser-visual-compare-layout-drift.
6. Run npm run test:browser-visual-compare-blocks-engine-adapter.
7. Confirm malformed values such as 1.5, 160px, zero, and negatives are rejected.
8. Confirm matrix entries accept maxExplanationElements, maxExplanationCandidates, and explainSelectors while retaining command-level selectors.

Verification

• TypeScript build/typecheck passes through the contract test
• All listed focused visual-compare tests pass
• Browser DOM capture test was not run locally because Playwright Chromium is not installed; screenshot-backed contract coverage passes

Backwards compatibility

Additive command-contract change. Existing defaults remain 25 explanation elements and 160 candidate DOM elements. No existing argument behavior changes except malformed positive-integer values now fail instead of being partially parsed.

AI assistance

• AI assistance: Yes
• Tool(s): OpenAI gpt-5.6-sol via OpenCode
• Used for: Implemented and reviewed the command contract, normalization, validation, evidence, tests, and PR.