Prepare plainforge release candidate surface

[rogerchappel]

Summary

• add an installed-tarball package smoke that imports plainforge and runs the packaged CLI
• verify convert and fixture inspection behavior from a temporary consumer project
• document package smoke coverage in the release readiness section

Verification

• npm run release:check
  • node --test passed: 13 tests
  • node --check passed for bin, src, and test files
  • scripts/build.js passed
  • scripts/smoke.sh passed
  • scripts/package-smoke.sh packed, installed, imported, converted, and inspected fixtures