fix(tui): select operator fields on mouse click

[edwin-zvs]

Summary

• record hit targets for visible operator definition rows
• update operator editor focus when a definition row is clicked
• cover all operator definition rows with a mouse regression test

Testing

• cargo test -p construct-cli operator_view_ (22 passed)
• cargo build
• cargo test -p construct-cli (1409 passed, 1 existing failure: color::tests::truecolor_backend_still_emits_rgb; reproduced on unchanged main)