rx.Field setter decorators with frontend compilation support #6230

Summary

Implement @field_name.setter decorator on rx.Field descriptors that auto-generates an event handler to set the var. For simple cases, rx.Field(..., setter=True) creates a default setter that assigns the value directly.

Frontend Compilation

Because these setters have trivial, known semantics (assign a value to a field), they can be compiled to JS that runs on the frontend. This is the bridge to Replicache optimistic mutations (1b-ii) — the frontend applies the setter immediately as a local mutation while the backend mutation is in flight. The user sees instant feedback with no round-trip.

For custom setters (@field_name.setter decorated functions), the same pattern applies if the setter body is simple enough to cross-compile. More complex setters fall back to backend-only execution with the optimistic mutation reverted on conflict.

Related

Addresses #3772 (rx.var / property setters).

Part of the Reflex Framework Roadmap — Section 1k.