feat(kernel): retried stages receive explicit failure feedback; default retry backoff 1s

core/events/src/main/kotlin/com/correx/core/events/execution/RetryPolicy.kt

@@ -5,7 +5,7 @@ import kotlinx.serialization.Serializable
 @Serializable
 data class RetryPolicy(
     val maxAttempts: Int,
-    val backoffMs: Long = 0L,
+    val backoffMs: Long = 1_000L,
 ) {
     init {
         require(maxAttempts >= 1) { "maxAttempts must be >= 1" }