docs: clarify Mini Program package imports

[coderwhy]

Summary

• document the supported CommonJS package-name import flow for WeChat Mini Programs
• explain the DevTools Build npm step and discourage source-file imports
• add a package-root entry-point regression test

Validation

• npm run test:types
• npm test (16 passing)
• npm run test:coverage (100% source coverage)
• npm audit --omit=dev --audit-level=high
• git diff --check
• remote CI will run npm pack --dry-run.