Fix npm packaging, TS test flow, and conduit subscribe ergonomics

[userFRM]

Summary

• make the npm package publish-safe by ensuring dist/* is included and checking packed contents in CI
• align local TypeScript testing with CI by using tsx via npm test
• make subscribe() validate channels up front and use per-channel events
• clean up README accuracy and tone around Tauri IPC

Verification

• cargo test
• cd packages/tauri-plugin-conduit && npm test
• cd packages/tauri-plugin-conduit && npm run check
• cd packages/tauri-plugin-conduit && npm run build && npm pack --dry-run 2>&1 | tee /tmp/tauri-plugin-conduit-pack.txt >/dev/null && grep 'dist/index.js' /tmp/tauri-plugin-conduit-pack.txt

Closes #13
Closes #14
Closes #15
Closes #16
Closes #17