on_connect lifecycle hook for @rx.event #6229

Summary

Replace on_hydrate and app-wide on_load with a first-class on_connect hook on the @rx.event decorator.

Details

When a client establishes a websocket connection, registered on_connect handlers fire automatically — no need for page-level on_load lists or the implicit on_hydrate mechanism. on_connect also fires on any initial session-establishing call.

This simplifies the lifecycle model:

• Explicit: no hidden on_hydrate magic
• Discoverable: registered in the same global event list as all other handlers
• Composable: multiple handlers, priority ordering, same semantics as other registered events

Works naturally with the EventHandler redesign (1b) — on_connect handlers are just event handlers with a lifecycle trigger.

Part of the Reflex Framework Roadmap — Section 1j.