fix(mat): never triage a survivor with a heartbeat as Deceased (safety) (#926)

Both triage paths in the Mass Casualty Assessment tool classified a
survivor as Deceased (Black) on "no breathing + no movement" while
completely ignoring the heartbeat signal:

- domain `TriageCalculator::calculate` → `combine_assessments(Absent, None)`
  returned Deceased. That branch is in fact only reachable *because* a
  heartbeat makes `has_vitals()` true (breathing+movement absent alone →
  Unknown) — so every "Deceased" was a live person with a pulse.
- detection `EnsembleClassifier::determine_triage` (the path used by
  `classify()`) returned Deceased on `!has_breathing && !has_movement`,
  also ignoring `reading.heartbeat`.

A survivor with a detectable pulse but no sensed breathing/movement is in
respiratory arrest — the most time-critical *savable* state. Reporting them
Deceased would deprioritize a rescuable person. WiFi-CSI also cannot confirm
death (no airway-repositioning step), so a pulse must override.

Fix: in both paths, if the result would be Deceased but a heartbeat is
present, return Immediate. Total absence of breathing, movement AND heartbeat
is unchanged (domain → Unknown, ensemble → Deceased).

2 safety regression tests added. Full MAT suite: 168 + 6 + 3 passed, 0 failed
(existing test_no_vitals_is_deceased still green — no heartbeat → Deceased).

v2/crates/wifi-densepose-mat/src/detection/ensemble.rs

@@ -172,6 +172,14 @@ impl EnsembleClassifier {
         let has_movement = reading.movement.movement_type != MovementType::None;
 
         if !has_breathing && !has_movement {
+            // SAFETY: a detectable heartbeat means the survivor is ALIVE. No
+            // sensed breathing/movement *with* a pulse is respiratory arrest —
+            // the most time-critical savable state (Immediate), never Deceased.
+            // Only the total absence of breathing, movement AND heartbeat is
+            // reported Deceased.
+            if reading.heartbeat.is_some() {
+                return TriageStatus::Immediate;
+            }
             return TriageStatus::Deceased;
         }
 
@@ -295,6 +303,27 @@ mod tests {
         assert_eq!(result.recommended_triage, TriageStatus::Deceased);
     }
 
+    /// SAFETY regression: heartbeat present but no sensed breathing/movement is
+    /// respiratory arrest — Immediate, never Deceased. Only the *total* absence
+    /// of breathing, movement AND heartbeat (the test above) is Deceased.
+    #[test]
+    fn test_heartbeat_with_no_breathing_or_movement_is_immediate() {
+        // breathing: None, heartbeat: Some(72 bpm), movement: None
+        let reading = make_reading(None, Some(72.0), MovementType::None);
+
+        let classifier = EnsembleClassifier::new(EnsembleConfig {
+            min_ensemble_confidence: 0.0,
+            ..EnsembleConfig::default()
+        });
+
+        let result = classifier.classify(&reading);
+        assert_eq!(
+            result.recommended_triage,
+            TriageStatus::Immediate,
+            "a survivor with a pulse must never be triaged Deceased"
+        );
+    }
+
     #[test]
     fn test_ensemble_confidence_weighting() {
         let classifier = EnsembleClassifier::new(EnsembleConfig {

v2/crates/wifi-densepose-mat/src/domain/triage.rs

@@ -104,7 +104,20 @@ impl TriageCalculator {
         let movement_status = Self::assess_movement(vitals);
 
         // Step 4: Combine assessments
-        Self::combine_assessments(breathing_status, movement_status)
+        let status = Self::combine_assessments(breathing_status, movement_status);
+
+        // Step 5: SAFETY OVERRIDE — a detectable heartbeat means the survivor is
+        // ALIVE. `combine_assessments` only sees breathing + movement, so a
+        // person with a pulse but no *sensed* breathing/movement (respiratory
+        // arrest, or breathing too shallow for CSI to pick up) would otherwise
+        // be reported Deceased and deprioritized for rescue. No breathing + a
+        // pulse is the most time-critical *savable* state, so escalate to
+        // Immediate rather than ever calling a survivor with a heartbeat dead.
+        if status == TriageStatus::Deceased && vitals.heartbeat.is_some() {
+            return TriageStatus::Immediate;
+        }
+
+        status
     }
 
     /// Assess breathing status
@@ -217,7 +230,9 @@ enum MovementAssessment {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::domain::{BreathingPattern, ConfidenceScore, MovementProfile};
+    use crate::domain::{
+        BreathingPattern, ConfidenceScore, HeartbeatSignature, MovementProfile, SignalStrength,
+    };
     use chrono::Utc;
 
     fn create_vitals(
@@ -233,6 +248,29 @@ mod tests {
         }
     }
 
+    /// SAFETY regression: a survivor with a detectable heartbeat but no sensed
+    /// breathing or movement is in respiratory arrest — Immediate (Red), and
+    /// must NEVER be reported Deceased. (Before the fix, `combine_assessments`
+    /// ignored heartbeat and returned Deceased; that path was in fact only
+    /// reachable *because* a heartbeat made `has_vitals()` true.)
+    #[test]
+    fn heartbeat_with_no_breathing_or_movement_is_immediate_not_deceased() {
+        let vitals = VitalSignsReading {
+            breathing: None,
+            heartbeat: Some(HeartbeatSignature {
+                rate_bpm: 72.0,
+                variability: 0.1,
+                strength: SignalStrength::Moderate,
+            }),
+            movement: MovementProfile::default(),
+            timestamp: Utc::now(),
+            confidence: ConfidenceScore::new(0.8),
+        };
+        let status = TriageCalculator::calculate(&vitals);
+        assert_eq!(status, TriageStatus::Immediate, "pulse present ⇒ alive");
+        assert_ne!(status, TriageStatus::Deceased);
+    }
+
     #[test]
     fn test_no_vitals_is_unknown() {
         let vitals = create_vitals(None, MovementProfile::default());