test(derive): cover handler compile failures

Add a fixture-backed compile-fail test for generated slhx handler contracts: unknown handles and invalid handler shape fail during cargo check with actionable diagnostics.

req: derive_handler/001

req: test/003

slhx-derive/tests/compile_fail.rs

@@ -0,0 +1,106 @@
+use std::path::{Path, PathBuf};
+use std::process::Command;
+
+#[test]
+fn handler_macro_reports_unknown_handle_and_bad_shape() {
+    // req: derive_handler/001 req: test/003
+    let fixture = Fixture::new("slhx-derive-handler-fail");
+    fixture.write(
+        "Cargo.toml",
+        &format!(
+            r#"[package]
+name = "slhx-derive-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::known\tknown\t1\n",
+    )
+    .unwrap();
+}
+"#,
+    );
+    fixture.write(
+        "src/lib.rs",
+        r#"#[slhx::handler]
+fn missing() -> impl slhx::IntoEffect {
+    slhx::EffectBatch::default()
+}
+
+#[slhx::handler]
+fn known() {}
+"#,
+    );
+
+    let output = Command::new("cargo")
+        .arg("check")
+        .arg("--quiet")
+        .arg("--manifest-path")
+        .arg(fixture.path.join("Cargo.toml"))
+        .env("CARGO_TARGET_DIR", fixture.path.join("target"))
+        .output()
+        .expect("cargo check fixture runs");
+
+    assert!(!output.status.success(), "fixture unexpectedly compiled");
+    let stderr = String::from_utf8_lossy(&output.stderr);
+    assert!(
+        stderr.contains("unknown slhx handle `missing`; add `data-slhx-handle=\"missing\"`"),
+        "missing unknown-handle diagnostic in stderr:\n{stderr}"
+    );
+    assert!(
+        stderr.contains("slhx handler `known` must accept a form/context parameter or return a value implementing IntoEffect"),
+        "missing handler-shape diagnostic in stderr:\n{stderr}"
+    );
+}
+
+fn repo_path(crate_name: &str) -> PathBuf {
+    Path::new(env!("CARGO_MANIFEST_DIR"))
+        .parent()
+        .expect("crate has workspace parent")
+        .join(crate_name)
+}
+
+struct Fixture {
+    path: PathBuf,
+}
+
+impl Fixture {
+    fn new(name: &str) -> Self {
+        let path = std::env::temp_dir().join(format!(
+            "{name}-{}-{}",
+            std::process::id(),
+            std::thread::current().name().unwrap_or("test")
+        ));
+        let _ = std::fs::remove_dir_all(&path);
+        std::fs::create_dir_all(path.join("src")).unwrap();
+        Self { path }
+    }
+
+    fn write(&self, relative: &str, contents: &str) {
+        let path = self.path.join(relative);
+        if let Some(parent) = path.parent() {
+            std::fs::create_dir_all(parent).unwrap();
+        }
+        std::fs::write(path, contents).unwrap();
+    }
+}
+
+impl Drop for Fixture {
+    fn drop(&mut self) {
+        let _ = std::fs::remove_dir_all(&self.path);
+    }
+}