Add generic runtime reference index

[chubes4]

Summary

• Adds a generic files/runtime-reference-index.json artifact report for local references discovered in captured HTML, CSS, and Markdown.
• Reports likely entrypoints, source-to-target metadata, and present/missing local file references without product-specific semantics.
• Wires the report into artifact metadata, manifests, and smoke coverage.

Verification

• npm run build
• npm run runtime-reference-index-smoke
• npm run artifact-contract-smoke passed before the full-check rerun was interrupted.
• npm run browser-actions-artifact-smoke
• npm run check was started and passed build, WP plugin smoke, browser runtime operation, package distribution, artifact verifier, and artifact patch-apply before interruption during artifact-contract-smoke rerun.
• npm run browser-probe-artifact-smoke currently fails on an existing console fixture expectation where the captured log contains a CSP inline-script violation instead of wp-codebox fixture console error; this is unrelated to the reference-index change.

Closes #316

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Implemented the generic runtime reference index, smoke coverage, verification runs, and PR drafting. Chris remains responsible for review and merge.