#[react_message]Expand description
Turn a struct into a typed React message payload.
Applying #[react_message] derives serde::Deserialize and ts_rs::TS and
implements both bevy::ecs::event::Event and bevy_react::ReactPayload, so the
type can be registered with App::add_react_handler / add_react_message, routed
from a React emit(name, value) call, and exported to TypeScript via
App::export_react_typescript.
The emit name defaults to the struct name with its first letter lowercased
(Count → "count", PlayerScore → "playerScore"); override it with
#[react_message(name = "...")].
ⓘ
#[react_message]
struct Count(usize); // name = "count"
#[react_message(name = "hp")]
struct Health(u32); // name = "hp"