Bind changes:
- Unify _twoWayBind, _shorthandBind, and _radioBind into a single
  adapter-based _bind function
- Auto-detect element properties (value, checked, valueAsNumber,
  textContent for contenteditable, value for custom elements)
- Remove bare 'bind $var' shorthand (now requires 'bind $var to me')
- Move assignability validation to install time for element expressions
- Wrap bind install in try/catch for proper error reporting
- Update all existing tests, add 7 new tests, update docs

Rename always -> live:
- Rename feature keyword, source file, test file, and doc file
- Update all references across tests, docs, and playground