feat(derive): check form model structure

Add #[slhx::form] for user-authored form models, emit form field metadata into slhx.syms, and require form handlers to accept models that passed generated field compatibility checks.

req: form/001

req: form/004

req: form/006

req: codegen/004

slhx-core/src/lib.rs

@@ -849,6 +849,8 @@ impl<I> Handle<I> {
     }
 }
 
+pub trait FormModel {}
+
 #[derive(Debug, Eq, PartialEq, Hash)]
 pub struct Form<T> {
     id: ResourceId,