A single form field: a name, a shared string value, and validation
rules. Cloning a FormField is cheap and shares state (D116 Phase 28
Step 8) — every clone reads/writes the SAME underlying atoms, the same
“clone shares identity” convention EditController/ScrollController
already use in this codebase. This is what lets TextInput::field(f),
Form::field(f), and the app’s own submit-button closure all see the
same live value/touched/errors without any manual synchronization.