fix(derive): require generated handler symbols

Make #[slhx::handler] fail when slhx.syms is missing instead of silently skipping generated handle checks. Add compile-fail coverage with an actionable build.rs hint.

req: build/003

req: test/003

slhx-derive/tests/compile_fail.rs

@@ -60,6 +60,57 @@ fn known() {}
     );
 }
 
+#[test]
+fn handler_macro_reports_missing_syms() {
+    // req: build/003 req: test/003
+    let fixture = Fixture::new("slhx-derive-handler-syms-fail");
+    fixture.write(
+        "Cargo.toml",
+        &format!(
+            r#"[package]
+name = "slhx-derive-handler-syms-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() {
+    println!("cargo:rerun-if-changed=build.rs");
+}
+"#,
+    );
+    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] could not find"),
+        "missing handler syms diagnostic in stderr:\n{stderr}"
+    );
+    assert!(
+        stderr.contains("add slhx_build::app().run()? to build.rs"),
+        "missing build.rs hint in stderr:\n{stderr}"
+    );
+}
+
 #[test]
 fn form_handle_requires_form_parameter() {
     // req: form/004 req: form/006 req: test/003