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-build/src/lib.rs

@@ -591,6 +591,17 @@ fn __slhx_attr(tag: &str, attr: &str) -> Option<::std::string::String> {
         for (handle_ident, form_ident) in &self.handle_forms {
             out.push_str(&format!("handle_form\t{handle_ident}\t{form_ident}\n"));
         }
+        for form in self.forms.values() {
+            for control in &form.controls {
+                out.push_str(&format!(
+                    "form_field\t{}\t{}\t{}\t{}\n",
+                    form.resource.ident,
+                    control.name,
+                    control.required,
+                    form_control_is_multiple(&control.kind)
+                ));
+            }
+        }
         for (handle_ident, params) in &self.handle_params {
             for param in params {
                 out.push_str(&format!("handle_param\t{handle_ident}\t{param}\n"));
@@ -939,6 +950,10 @@ fn stable_id(kind: &str, symbol: &str) -> u32 {
     hash
 }
 
+fn form_control_is_multiple(kind: &ControlKind) -> bool {
+    matches!(kind, ControlKind::Select { multiple: true })
+}
+
 fn form_control_kind_expr(kind: &ControlKind) -> String {
     match kind {
         ControlKind::Text => "::slhx::FormControlKind::Text".to_string(),
@@ -1075,6 +1090,8 @@ mod tests {
 
         let syms = std::fs::read_to_string(out.join("slhx.syms")).unwrap();
         assert!(syms.contains("handle_form\tsave\tprofile\n"));
+        assert!(syms.contains("form_field\tprofile\ttitle\ttrue\tfalse\n"));
+        assert!(syms.contains("form_field\tprofile\tlabels\tfalse\ttrue\n"));
 
         let _ = std::fs::remove_dir_all(&base);
     }