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/src/lib.rs

@@ -4,12 +4,12 @@
 //! here, and import generated resources through `#[slhx::surface]`.
 
 pub use slhx_core::*;
-pub use slhx_derive::{app, component, handler, surface};
+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,
     };
-    pub use slhx_derive::{app, component, handler, surface};
+    pub use slhx_derive::{app, component, form, handler, surface};
 }