Relax Send bounds on single-threaded wasm targets

[sagikazarmark]

Problem

Connection is Send throughout the library:

• ConnectTo<R: Role>: Send + 'static
• fn connect_to(...) -> impl Future<Output = Result<()>> + Send
• Role: Debug + Clone + Send + Sync + 'static + Eq + Ord + Hash
• handler futures return impl Future<...> + Send
• SessionBlockState: Send + 'static + Sync
• BoxFuture (which is Send) rather than LocalBoxFuture at every boxed-future site

The doc comment on connect_to states it outright: "The future must be Send."

Since the wasm target gating landed, the crate compiles for wasm32-unknown-unknown, but browser runtimes are single-threaded and the surrounding types (web_sys::WebSocket, js_sys handles, Dioxus/Leptos/yew signals, Rc/RefCell) are !Send.

Impact

Adoption is possible but requires wrapping the SDK: handlers forward plain data into a futures::mpsc, and the !Send world drains it on the far side.

It works, but it is a layer added rather than a layer removed, which weakens the case for adopting the SDK over a hand-rolled client in the first place.

Proposal

Conditionally relax the bounds for WASM:

#[cfg(not(target_family = "wasm"))]
pub trait MaybeSend: Send {}
#[cfg(not(target_family = "wasm"))]
impl<T: Send> MaybeSend for T {}

#[cfg(target_family = "wasm")]
pub trait MaybeSend {}
#[cfg(target_family = "wasm")]
impl<T> MaybeSend for T {}

plus a MaybeSendBoxFuture<'a, T> alias.