Improve diffbudget release verification hygiene

[rogerchappel]

What changed

• Added or tightened package verification, local development docs, and npm package support metadata where this repo still had queued release-readiness gaps.
• Kept separate commits for CI, docs, and package metadata when multiple gaps were present.

Why it matters

The repo now has clearer install/test signals for contributors and stronger package contents for consumers.

Verification

• npm install --package-lock=false or npm ci: passed
• npm run check: passed
• npm test: passed
• npm pack --dry-run: passed

Remaining limitations

• No package was published and no release tag was created.