iter ms.1: pipeline.rs format!("check: {e}") → {e:#} preserves anyhow Caused-by chain in JSON-cohort feedback

Inline pinning test in pipeline.rs constructs the same chain
shape module_name_from_json produces on serde-parse failure
(leaf wrapped with "parsing JSON in <path>" context) and
asserts both layers survive the formatter. RED→GREEN against
the property, not against production line bytes — for a
two-character fix, extracting a helper just for testability
would be over-engineering.

The strip_locations regex collapses \nCaused by: chains; the
{:#} alternate Display joins with ': ' on one line, so the
collapser is a no-op on the fixed output. AILX cohort
unaffected — only the harness-side anyhow path (module
rename pre-check) was dropping cause information.

Harness suite: 14/14 green (was 13/13 + 1 new pin).

experiments/2026-05-12-cross-model-authoring/harness/src/pipeline.rs

@@ -115,7 +115,7 @@ pub fn run_pipeline(
         }
         Err(e) => {
             return Ok(PipelineCapture {
-                error: Some(format!("check: {e}")),
+                error: Some(format!("check: {e:#}")),
                 stdout: String::new(),
                 stderr: e.to_string(),
             });
@@ -217,3 +217,30 @@ fn run_with_timeout(bin: &Path, timeout: Duration) -> Result<RunOutput> {
         std::thread::sleep(Duration::from_millis(25));
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use anyhow::{anyhow, Context, Error};
+
+    /// Constructs the same chain shape `module_name_from_json`
+    /// produces on a serde-parse failure: a leaf error wrapped
+    /// with a "parsing JSON in <path>" context. Asserts that the
+    /// format string used at pipeline.rs error-emission preserves
+    /// both layers.
+    #[test]
+    fn check_format_preserves_anyhow_chain() {
+        let leaf: Error = anyhow!("expected value at line 1 column 1");
+        let chained = Err::<(), _>(leaf)
+            .context("parsing JSON in /tmp/x.ail.json")
+            .unwrap_err();
+        let formatted = format!("check: {chained:#}");
+        assert!(
+            formatted.contains("parsing JSON in /tmp/x.ail.json"),
+            "top-level context missing; got: {formatted}",
+        );
+        assert!(
+            formatted.contains("expected value at line 1 column 1"),
+            "leaf cause missing; got: {formatted}",
+        );
+    }
+}