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-core/src/lib.rs

@@ -849,6 +849,10 @@ impl<I> Handle<I> {
     }
 }
 
+pub trait FormValue {}
+
+impl<T> FormValue for T where T: std::str::FromStr {}
+
 pub trait FormModel {}
 
 #[derive(Debug, Eq, PartialEq, Hash)]