Iter 4a: ail check --json mit strukturierten Diagnostics

Neue Top-Level-API check_module(&Module) -> Vec<Diagnostic> in
ailang-check, plus stabile Codes (unbound-var, type-mismatch,
arity-mismatch, non-exhaustive-match, unknown-ctor-in-pattern,
duplicate-def, …). CLI bekommt --json-Flag für maschinenlesbares
Output, Exit 1 bei Errors. Text-Modus unverändert. E2E-Test
check_json_unbound_var sichert das Format ab.

crates/ail/tests/e2e.rs

@@ -63,3 +63,34 @@ fn list_sum_via_match() {
     let stdout = build_and_run("list.ail.json");
     assert_eq!(stdout.trim(), "42");
 }
+
+/// Schützt das `--json`-Diagnostic-Format für Tooling-Konsumenten.
+/// `broken_unbound.ail.json` referenziert eine nicht-existente Variable;
+/// erwartet wird Exit-Code 1 und mindestens ein Diagnostic mit
+/// `severity == "error"` und `code == "unbound-var"`.
+#[test]
+fn check_json_unbound_var() {
+    let manifest_dir = env!("CARGO_MANIFEST_DIR");
+    let workspace = Path::new(manifest_dir).parent().unwrap().parent().unwrap();
+    let src = workspace.join("examples").join("broken_unbound.ail.json");
+
+    let output = Command::new(ail_bin())
+        .args(["check", src.to_str().unwrap(), "--json"])
+        .output()
+        .expect("ail check --json failed to run");
+
+    let code = output.status.code().expect("process terminated by signal");
+    assert_eq!(code, 1, "expected exit code 1, stderr: {}", String::from_utf8_lossy(&output.stderr));
+
+    let stdout = String::from_utf8(output.stdout).expect("stdout utf8");
+    let diags: serde_json::Value =
+        serde_json::from_str(stdout.trim()).expect("stdout must be valid JSON");
+    let arr = diags.as_array().expect("diagnostics must be a JSON array");
+    assert!(
+        arr.iter().any(|d| {
+            d.get("severity").and_then(|v| v.as_str()) == Some("error")
+                && d.get("code").and_then(|v| v.as_str()) == Some("unbound-var")
+        }),
+        "expected at least one error diagnostic with code unbound-var; got: {stdout}"
+    );
+}