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/tests/compile_fail.rs

@@ -60,6 +60,58 @@ fn known() {}
     );
 }
 
+#[test]
+fn form_handle_requires_form_parameter() {
+    // req: form/004 req: form/006 req: test/003
+    let fixture = Fixture::new("slhx-derive-form-handler-fail");
+    fixture.write(
+        "Cargo.toml",
+        &format!(
+            r#"[package]
+name = "slhx-derive-form-handler-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#"#[slhx::handler]
+fn create() -> 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-handler diagnostic in stderr:\n{stderr}"
+    );
+}
+
 #[test]
 fn surface_macro_reports_missing_generated_include() {
     // req: build/004 req: test/003