fix(runner): contain mc synthetic member panics (GREEN)

blueprint_mc_family joins its sweep/walkforward siblings on the
51096a3 machinery: catch_member_panic around the bare member run in
the monte_carlo draw closure, per-seed capture, lowest-seed
resolution after the join (seed-input order, thread-order-independent,
C1), exit via the shared aura: warning: + exit 3 path. Ordering is
load-bearing and documented at the site: the captured fault resolves
BEFORE the vacuous-mc guard, so metrics-identical placeholder reports
from faulted draws cannot masquerade as a vacuous run. All three
containment e2es green; clippy clean. refs #278

crates/aura-runner/src/family.rs

@@ -354,6 +354,20 @@ fn lowest_point_fault(family: &SweepFamily, faults: Mutex<Vec<(Vec<Cell>, String
         .find_map(|pt| captured.iter().find(|(p, _)| *p == pt.params).map(|(_, m)| m.clone()))
 }
 
+/// The LOWEST seed's captured member-panic message (#278), found by walking
+/// `family.draws` — `monte_carlo`'s own contract guarantees seed-**input**
+/// order regardless of thread completion (C1) — for the first draw whose
+/// seed matches a captured fault. Thread-order-independent by construction,
+/// mirroring [`lowest_point_fault`]'s and `lowest_window_fault`'s identical
+/// convention one level over (per-point sweep / per-window walk-forward).
+fn lowest_seed_fault(family: &McFamily, faults: Mutex<Vec<(u64, String)>>) -> Option<String> {
+    let captured = faults.into_inner().expect("fault capture lock");
+    family
+        .draws
+        .iter()
+        .find_map(|d| captured.iter().find(|(s, _)| *s == d.seed).map(|(_, m)| m.clone()))
+}
+
 /// Render + exit(3) on a contained member panic (#278: the deliberate
 /// failed-cells exit, C14, never the raw 101 an uncontained panic would
 /// yield) — reuses `aura_campaign::member_fault_prose`'s established
@@ -772,11 +786,35 @@ pub fn blueprint_mc_family(
     // re-runs the shared reduce-mode member path over its own seeded synthetic walk.
     let seeds: Vec<u64> = (1..=n_seeds).collect();
     let base_point: Vec<Scalar> = Vec::new();
+    // #278: `run_blueprint_member` ran bare here, the one family builder without
+    // the #272 fault boundary its sweep/walk-forward siblings gained in 51096a3
+    // — a member-compile panic (e.g. an `Sma::new` length assert) would otherwise
+    // unwind straight through `monte_carlo`'s `run_indexed` to an uncaught exit
+    // 101. Contained the same way: `catch_member_panic` + a per-seed capture,
+    // resolved to the LOWEST seed's message after the join (thread-order-
+    // independent, C1) via `lowest_seed_fault`.
+    let faults: Mutex<Vec<(u64, String)>> = Mutex::new(Vec::new());
     let family = monte_carlo(&base_point, &seeds, |seed, _base| {
         let sources = synthetic_walk_sources(seed);
         let window = window_of(&sources).expect("non-empty synthetic walk");
-        run_blueprint_member(reload(doc), &[], &space, sources, window, seed, pip, &topo, env, DEFAULT_STOP, &binding, &[], NO_INSTRUMENT_CONTEXT)
+        match catch_member_panic(|| {
+            run_blueprint_member(reload(doc), &[], &space, sources, window, seed, pip, &topo, env, DEFAULT_STOP, &binding, &[], NO_INSTRUMENT_CONTEXT)
+        }) {
+            Ok(report) => report,
+            Err(msg) => {
+                faults.lock().expect("fault capture lock").push((seed, msg));
+                axis_grid_probe_report()
+            }
+        }
     });
+    // The captured fault must win BEFORE the vacuous-mc guard below: a faulted
+    // draw's placeholder report is metrics-identical across every faulted seed,
+    // so a run with >= 2 faulted seeds (or one faulted + one real draw sharing
+    // its placeholder's zero metrics) could otherwise trip the vacuous refusal
+    // instead of surfacing the real member fault.
+    if let Some(msg) = lowest_seed_fault(&family, faults) {
+        exit_on_member_panic(&msg);
+    }
     // Silent-vacuous MC guard (refuse-don't-guess, C10): with >= 2 seeds, if every draw's
     // metrics are bit-identical to the first, no seed reached a distinguishable realization —
     // the strategy never warmed over the fixed synthetic walk (e.g. a lookback as deep as the