feat(derive): require form parameter for form handles

Emit handle-form facts from slhx-build and have #[slhx::handler] reject generated form handlers that do not accept slhx::Form<_>. Cover the boundary with compile-fail fixtures.

req: form/004

req: form/006

req: test/003

slhx-derive/src/lib.rs

@@ -30,6 +30,16 @@ pub fn handler(_attr: TokenStream, item: TokenStream) -> TokenStream {
                 )
                 .into();
             }
+            if handle_requires_form(&syms_path, &name) && !has_form_param(&function) {
+                let message = format!(
+                    "slhx handler `{name}` handles a generated form and must accept slhx::Form<_>"
+                );
+                return quote!(
+                    #function
+                    compile_error!(#message);
+                )
+                .into();
+            }
         }
     }
 
@@ -125,6 +135,17 @@ fn syms_contains_handle(path: &PathBuf, ident: &str) -> bool {
     })
 }
 
+fn handle_requires_form(path: &PathBuf, ident: &str) -> bool {
+    let Ok(syms) = std::fs::read_to_string(path) else {
+        return false;
+    };
+    syms.lines().any(|line| {
+        let mut fields = line.split('\t');
+        matches!(fields.next(), Some("handle_form"))
+            && fields.next().is_some_and(|handle_ident| handle_ident == ident)
+    })
+}
+
 fn compile_error(message: &str) -> TokenStream {
     format!("compile_error!({message:?});")
         .parse()
@@ -133,7 +154,7 @@ fn compile_error(message: &str) -> TokenStream {
 
 #[cfg(test)]
 mod tests {
-    use super::{has_form_param, has_non_unit_return, syms_contains_handle};
+    use super::{handle_requires_form, has_form_param, has_non_unit_return, syms_contains_handle};
     use syn::parse_quote;
 
     #[test]
@@ -141,12 +162,14 @@ mod tests {
         let path = std::env::temp_dir().join("slhx-derive-syms-test.syms");
         std::fs::write(
             &path,
-            "slhx-syms-v1\nslot\ttemplates/a.heml::count\tcount\t1\nhandle\ttemplates/a.heml::create\tcreate\t2\n",
+            "slhx-syms-v1\nslot\ttemplates/a.heml::count\tcount\t1\nhandle\ttemplates/a.heml::create\tcreate\t2\nhandle_form\tcreate\tnew_todo\n",
         )
         .unwrap();
 
         assert!(syms_contains_handle(&path, "create"));
         assert!(!syms_contains_handle(&path, "missing"));
+        assert!(handle_requires_form(&path, "create"));
+        assert!(!handle_requires_form(&path, "missing"));
 
         let _ = std::fs::remove_file(path);
     }