feat: add deterministic execution heartbeats (AEG-V15-038)

Adds a pure, monotonic execution heartbeat and caller-supplied staleness cutoff without inventing alert thresholds. Evidence: targeted Release tests 5/5 passed; TRX SHA256 2ADBB526FAF6E5D924EB3F53C7E582E736199A59E0DA4E4FD25DCAA82A661BBC. WBS remains IN_PROGRESS pending approved alert contract.

src/KArtSell.Modules.ModelOperations/Domain/ModelOperationExecution.cs

@@ -48,6 +48,7 @@ public sealed class ModelOperationExecution
     public string IdempotencyKey { get; }
     public DateTimeOffset RequestedAt { get; }
     public DateTimeOffset LastOccurredAt { get; private set; }
+    public DateTimeOffset? LastHeartbeatAt { get; private set; }
     public ModelOperationExecutionState State { get; private set; }
     public DateTimeOffset? HoldUntil { get; private set; }
     public IReadOnlyList<ModelOperationExecutionTransition> Transitions => transitions;
@@ -73,8 +74,28 @@ public sealed class ModelOperationExecution
         State = next;
         LastOccurredAt = occurredAt;
         HoldUntil = holdUntil;
+        if (next == ModelOperationExecutionState.Running) LastHeartbeatAt = occurredAt;
         return transition;
     }
 
+    public void RecordHeartbeat(DateTimeOffset occurredAt)
+    {
+        if (State != ModelOperationExecutionState.Running)
+            throw new InvalidOperationException("Only a running execution may record a heartbeat.");
+        if (occurredAt < LastHeartbeatAt)
+            throw new InvalidOperationException("Execution heartbeat time cannot move backwards.");
+
+        LastHeartbeatAt = occurredAt;
+    }
+
+    public bool HasExceededHeartbeatCutoff(DateTimeOffset asOf, TimeSpan allowedSilence)
+    {
+        ArgumentOutOfRangeException.ThrowIfLessThan(allowedSilence, TimeSpan.Zero);
+        if (asOf < LastHeartbeatAt) throw new InvalidOperationException("Heartbeat cutoff cannot precede the latest heartbeat.");
+        return State == ModelOperationExecutionState.Running
+            && LastHeartbeatAt.HasValue
+            && asOf - LastHeartbeatAt.Value > allowedSilence;
+    }
+
     private static HashSet<ModelOperationExecutionState> Set(params ModelOperationExecutionState[] states) => new(states);
 }