test(cli): RED — sweep --real --trace refuses instead of writing tap series

Pins the #224 delivery headline: the #168 refusal lifts and at least
one non-empty tap-series file lands under runs/traces/. Layout-agnostic
on purpose — the per-member key convention is the GREEN slice's call.

refs #224

crates/aura-cli/tests/cli_run.rs

@@ -1225,6 +1225,82 @@ fn sweep_real_blueprint_member_lines_pin_the_dissolved_contract() {
     assert_eq!(count("campaigns"), 1, "second identical run dedupes the campaign doc");
 }
 
+/// True iff `root` contains, at any depth, a regular file of non-zero length —
+/// the layout-agnostic "traces actually landed on disk" probe. #224 does not yet
+/// pin the per-member trace-directory naming (the sweep sugar's cell has no
+/// nominee and `persist_campaign_traces` keys by `<trace_name>/<cell_key>`), so
+/// this walks the tree rather than hard-coding a path, keeping the headline about
+/// *that* traces are written, not *where*.
+fn any_nonempty_file(root: &Path) -> bool {
+    let mut stack = vec![root.to_path_buf()];
+    while let Some(p) = stack.pop() {
+        let Ok(entries) = std::fs::read_dir(&p) else { continue };
+        for entry in entries.flatten() {
+            let path = entry.path();
+            if path.is_dir() {
+                stack.push(path);
+            } else if std::fs::metadata(&path).map(|m| m.len() > 0).unwrap_or(false) {
+                return true;
+            }
+        }
+    }
+    false
+}
+
+/// Property (#224): `aura sweep <blueprint.json> --real SYM --axis … --trace <fam>`
+/// DELIVERS the advertised trace-writing capability instead of the #168
+/// refuse-with-pointer. Two things must hold once the capability ships:
+/// (1) the run no longer refuses (`aura: --trace is not yet available on sweep;
+/// see #224`, exit 2) but succeeds, and (2) the tap series actually land on disk
+/// under `runs/traces/` — at least one non-empty tap file is written, the honest
+/// inverse of the #168 no-op where the `--real` sugar set `persist_taps: vec![]`
+/// (verb_sugar.rs) and `run_blueprint_sweep` did `let _ = persist`, so nothing was
+/// ever written. The exact on-disk layout (trace name + per-member/nominee key)
+/// is deliberately NOT pinned here: this headline pins that traces are written at
+/// all — not how many dirs, nor under what member key — because that naming is
+/// the GREEN slice's open design point (the selection-free sweep cell has no
+/// nominee for the existing per-cell `persist_campaign_traces` mechanism, #224).
+/// Gated on the local GER40 archive (the project skip-on-no-data convention);
+/// the refused status quo makes it RED on a data-ful host today.
+#[test]
+fn sweep_real_with_trace_writes_tap_series_to_disk() {
+    if !local_data_present() {
+        eprintln!("skip: no local data at {}", data_server::DEFAULT_DATA_PATH);
+        return;
+    }
+    let (dir, _g) = fresh_project();
+    let fixture = format!("{}/examples/r_sma_open.json", env!("CARGO_MANIFEST_DIR"));
+    let out = std::process::Command::new(BIN)
+        .args([
+            "sweep", &fixture,
+            "--real", "GER40",
+            "--from", GER40_SEPT2024_FROM_MS,
+            "--to", GER40_SEPT2024_TO_MS,
+            "--axis", "sma_signal.fast.length=2,4",
+            "--trace", "brp",
+        ])
+        .current_dir(dir)
+        .output()
+        .unwrap();
+    // (1) the #168 refusal is lifted: the advertised flag runs instead of exit 2.
+    assert!(
+        out.status.success(),
+        "sweep --trace must no longer refuse (#224); exit {:?}, stderr: {}",
+        out.status.code(),
+        String::from_utf8_lossy(&out.stderr),
+    );
+    // (2) the tap series actually land on disk under runs/traces/ (non-empty) —
+    //     the honest inverse of the #168 `persist_taps: vec![]` no-op.
+    let traces_root = dir.join("runs").join("traces");
+    assert!(
+        any_nonempty_file(&traces_root),
+        "sweep --trace must write at least one non-empty tap file under {} \
+         (found none — the #168 no-op wrote nothing); stderr: {}",
+        traces_root.display(),
+        String::from_utf8_lossy(&out.stderr),
+    );
+}
+
 /// Extract the first persisted family whose `kind` matches `want` from
 /// `aura runs families` stdout (one JSON object per line), returning its
 /// `family_id` — the id `aura reproduce` addresses. Panics if none matches so a