[codex] Fix CommonJS package entrypoint

[seveibar]

Summary

• Point main at the emitted UMD bundle, dist/eecircuit-engine.umd.js.
• Point exports['.'].require at the same emitted CommonJS-compatible bundle.

Root cause

The package publishes dist/eecircuit-engine.umd.js, but package metadata referenced dist/eecircuit-engine.umd.cjs and dist/eecircuit-engine.cjs, which are not included in the tarball.

Validation

• Packed the package with the existing published dist artifacts present.
• Installed the tarball into a throwaway consumer project.
• Verified require('@tscircuit/eecircuit-engine') resolves.
• Verified import('@tscircuit/eecircuit-engine') still resolves.

Note: local npm install emitted engine warnings because this shell is on Node 22.3.0 while current Vite/ESLint packages require newer Node 22 or compatible Node 20/24.