feat(derive): check component handlers

Make #[slhx::component] compare generated handle symbols with inline #[slhx::handler] functions and fail when a required handler implementation is missing.

req: component/005

req: check/002

req: test/003

slhx-derive/tests/compile_fail.rs

@@ -1,6 +1,61 @@
 use std::path::{Path, PathBuf};
 use std::process::Command;
 
+#[test]
+fn component_macro_reports_missing_handler_implementation() {
+    // req: component/005 req: check/002 req: test/003
+    let fixture = Fixture::new("slhx-derive-component-missing-handler-fail");
+    fixture.write(
+        "Cargo.toml",
+        &format!(
+            r#"[package]
+name = "slhx-derive-component-missing-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\ttemplates/app.heml::delete\tdelete\t2\n",
+    )
+    .unwrap();
+}
+"#,
+    );
+    fixture.write(
+        "src/lib.rs",
+        r#"#[slhx::component]
+mod todos {
+    #[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::component] missing handler implementation(s): delete"),
+        "missing component diagnostic in stderr:\n{stderr}"
+    );
+}
+
 #[test]
 fn handler_macro_reports_unknown_handle_and_bad_shape() {
     // req: derive_handler/001 req: test/003