Add burn rate calculation helper

slo_tracker.py

@@ -62,3 +62,11 @@ def main():
 
 if __name__ == "__main__":
     main()
+
+
+def burn_rate(error_rate: float, window_hours: float, target: float) -> float:
+    """Calculate burn rate: how fast the error budget is being consumed."""
+    budget = 1.0 - target
+    if budget == 0:
+        return float("inf")
+    return error_rate / budget * (window_hours / (30 * 24))