Strengthen package smoke assertions

[rogerchappel]

Summary

• replace raw npm pack dry-run with a package smoke validator
• assert the CLI, library entrypoint, and support docs are present in the published tarball
• document the package smoke guarantee in the release-readiness section

Verification

• npm ci
• npm run package:smoke (Package smoke passed with 49 files.)
• npm run release:check (11 tests passed; smoke ok; package smoke passed with 49 files.)

Risk Level

Low. Release-readiness script and docs only; runtime code is unchanged.

Rollback Plan

Revert this PR to restore the previous raw npm pack dry-run package smoke.