fix(derive): reject form type impostors

Tighten generated form-handler validation so only actual Form paths satisfy form handles; names like CreateForm no longer bypass the slhx::Form<_> requirement. Cover with unit and compile-fail tests.

req: form/004

req: form/006

req: test/003

slhx-derive/src/lib.rs

@@ -120,11 +120,18 @@ fn generated_path(file: &str) -> Option<PathBuf> {
 
 fn has_form_param(function: &ItemFn) -> bool {
     function.sig.inputs.iter().any(|arg| match arg {
-        FnArg::Typed(arg) => type_ends_with(&arg.ty, "Form"),
+        FnArg::Typed(arg) => is_form_type(&arg.ty),
         FnArg::Receiver(_) => false,
     })
 }
 
+fn is_form_type(ty: &Type) -> bool {
+    match ty {
+        Type::Path(path) => path.path.segments.last().is_some_and(|segment| segment.ident == "Form"),
+        _ => false,
+    }
+}
+
 fn has_non_unit_return(function: &ItemFn) -> bool {
     match &function.sig.output {
         ReturnType::Default => false,
@@ -132,13 +139,6 @@ fn has_non_unit_return(function: &ItemFn) -> bool {
     }
 }
 
-fn type_ends_with(ty: &Type, ident: &str) -> bool {
-    match ty {
-        Type::Path(path) => path.path.segments.last().is_some_and(|segment| segment.ident == ident),
-        _ => false,
-    }
-}
-
 fn syms_contains_handle(path: &PathBuf, ident: &str) -> bool {
     let Ok(syms) = std::fs::read_to_string(path) else {
         return true;
@@ -169,8 +169,8 @@ fn compile_error(message: &str) -> TokenStream {
 
 #[cfg(test)]
 mod tests {
-    use super::{handle_requires_form, has_form_param, has_non_unit_return, syms_contains_handle};
+    use super::{handle_requires_form, has_form_param, has_non_unit_return, is_form_type, syms_contains_handle};
-    use syn::parse_quote;
+    use syn::{parse_quote, Type};
 
     #[test]
     fn syms_lookup_matches_handle_ident() {
@@ -201,4 +201,13 @@ mod tests {
         assert!(!has_form_param(&empty));
         assert!(!has_non_unit_return(&empty));
     }
+
+    #[test]
+    fn form_param_matches_form_type_not_name_suffix() {
+        let real_form: Type = parse_quote!(slhx::Form<CreateTodo>);
+        let impostor: Type = parse_quote!(CreateTodoForm);
+
+        assert!(is_form_type(&real_form));
+        assert!(!is_form_type(&impostor));
+    }
 }

slhx-derive/tests/compile_fail.rs

@@ -163,6 +163,60 @@ fn create() -> impl slhx::IntoEffect {
     );
 }
 
+#[test]
+fn form_handle_rejects_form_name_suffix_impostor() {
+    // req: form/004 req: form/006 req: test/003
+    let fixture = Fixture::new("slhx-derive-form-impostor-fail");
+    fixture.write(
+        "Cargo.toml",
+        &format!(
+            r#"[package]
+name = "slhx-derive-form-impostor-fail"
+version = "0.0.0"
+edition = "2021"
+
+[lib]
+path = "src/lib.rs"
+
+[dependencies]
+slhx = {{ path = {:?} }}
+"#,
+            repo_path("slhx")
+        ),
+    );
+    fixture.write(
+        "build.rs",
+        r#"fn main() {
+    let out = std::path::PathBuf::from(std::env::var_os("OUT_DIR").unwrap());
+    std::fs::write(
+        out.join("slhx.syms"),
+        "slhx-syms-v1\nhandle\ttemplates/app.heml::create\tcreate\t1\nhandle_form\tcreate\tnew_todo\n",
+    )
+    .unwrap();
+}
+"#,
+    );
+    fixture.write(
+        "src/lib.rs",
+        r#"struct CreateForm;
+
+#[slhx::handler]
+fn create(_form: CreateForm) -> impl slhx::IntoEffect {
+    slhx::EffectBatch::default()
+}
+"#,
+    );
+
+    let output = check_fixture(&fixture);
+
+    assert!(!output.status.success(), "fixture unexpectedly compiled");
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert!(
+        stderr.contains("slhx handler `create` handles a generated form and must accept slhx::Form<_>"),
+        "missing form-impostor diagnostic in stderr:\n{stderr}"
+    );
+}
+
 #[test]
 fn surface_macro_reports_missing_generated_include() {
     // req: build/004 req: test/003