Improve prpack release check coverage

[rogerchappel]

What changed

• Hardened release:check so the release dry run exercises typecheck/check, tests, smoke coverage, and package packing through the canonical npm script.
• Kept the change scoped to release readiness; no runtime behavior changed.

Why it matters

This makes PR and release evidence harder to accidentally weaken. A package can no longer pass the release dry run while skipping the local validation developers rely on before publishing.

Verification

• npm ci --no-audit --no-fund passed.
• npm run release:check passed.
• bash scripts/validate.sh passed.

Remaining limitations

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