test(diagnostics): require accurate causes and source lines

Check native and IR rejection without creating or replacing output files.
Keep nine missing-location failures visible rather than accepting them.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/tests/iii/multiple-errors/case.json b/tests/iii/multiple-errors/case.json
new file mode 100644
index 0000000..32ec325
--- /dev/null
+++ b/tests/iii/multiple-errors/case.json
@@ -0,0 +1,12 @@
+{
+  "errors": [
+    {
+      "reason": "Unterminated\\ string\\ literal",
+      "line": 1
+    },
+    {
+      "reason": "Expected\\ ':'\\ after\\ variable\\ name",
+      "line": 2
+    }
+  ]
+}