feat(context,kernel): compression pipeline stage 3 — replay-safe tier summarizer

ContextSummarizedEvent (registered) keyed on stable session sequence, not the
ephemeral pack ordinal, so replay reuses the recorded summary artifact and never
re-calls the LLM (invariants #6/#8). TierContextSummarizer mirrors
JournalCompactionService for freeform context (tool outputs/docs/retrieved text)
the journal path doesn't cover. ContextState + DefaultContextReducer fold the
event; reducer replay-safety unit-tested.

core/events/src/main/kotlin/com/correx/core/events/events/ContextSummarizedEvent.kt

@@ -0,0 +1,23 @@
+package com.correx.core.events.events
+
+import com.correx.core.events.types.ArtifactId
+import com.correx.core.events.types.SessionId
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+
+/**
+ * Records that freeform context up through session sequence [coversThroughSequence] was
+ * summarized into [summaryArtifactId] at tier [tier]. Keyed on the stable event sequence (not
+ * the ephemeral per-assembly pack ordinal, which is re-stamped each build) so replay reuses
+ * the recorded summary deterministically and never re-calls the LLM (invariants #6/#8).
+ * [tier] 2 = light single-pass summary, 3 = summary-of-summaries.
+ */
+@Serializable
+@SerialName("ContextSummarized")
+data class ContextSummarizedEvent(
+    val sessionId: SessionId,
+    val coversThroughSequence: Long,
+    val summaryArtifactId: ArtifactId,
+    val tier: Int,
+    val entriesOmittedCount: Int,
+) : EventPayload

core/events/src/main/kotlin/com/correx/core/events/serialization/Serialization.kt

@@ -61,6 +61,7 @@ import com.correx.core.events.events.StageCompletedEvent
 import com.correx.core.events.events.PreemptRedirectBlockedEvent
 import com.correx.core.events.events.PreemptRedirectEvent
 import com.correx.core.events.events.StageFailedEvent
+import com.correx.core.events.events.ContextSummarizedEvent
 import com.correx.core.events.events.SteeringNoteAddedEvent
 import com.correx.core.events.events.ToolCallAssessedEvent
 import com.correx.core.events.events.ToolExecutionCompletedEvent
@@ -107,6 +108,7 @@ val eventModule = SerializersModule {
         subclass(ToolCallAssessedEvent::class)
         subclass(FileWrittenEvent::class)
         subclass(SteeringNoteAddedEvent::class)
+        subclass(ContextSummarizedEvent::class)
         subclass(PreemptRedirectEvent::class)
         subclass(PreemptRedirectBlockedEvent::class)
         subclass(InitialIntentEvent::class)