feat(derive): check form parser availability

Introduce FormValue as the parser-availability bound for #[slhx::form] models, blanket it for FromStr types, and add compile-fail coverage for a form field whose domain type lacks a parser.

req: form/004

req: form/006

slhx/src/lib.rs

@@ -9,7 +9,7 @@ pub use slhx_derive::{app, component, form, handler, surface};
 pub mod prelude {
     pub use slhx_core::{
         navigate, push, redirect, replace, Atom, BuildFingerprint, CssClass, CssClasses, Effect,
-        Form, Handle, IntoEffect, KeyedSlot, Slot,
+        Form, FormModel, FormValue, Handle, IntoEffect, KeyedSlot, Slot,
     };
     pub use slhx_derive::{app, component, form, handler, surface};
 }