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/context/src/test/kotlin/ContextSummaryReducerTest.kt

@@ -0,0 +1,45 @@
+import com.correx.core.context.DefaultContextReducer
+import com.correx.core.context.state.ContextState
+import com.correx.core.events.events.ContextSummarizedEvent
+import com.correx.core.events.events.EventMetadata
+import com.correx.core.events.events.StoredEvent
+import com.correx.core.events.types.ArtifactId
+import com.correx.core.events.types.EventId
+import com.correx.core.events.types.SessionId
+import kotlinx.datetime.Instant
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Test
+
+class ContextSummaryReducerTest {
+
+    private val reducer = DefaultContextReducer()
+
+    private fun stored(payload: ContextSummarizedEvent, seq: Long) = StoredEvent(
+        metadata = EventMetadata(
+            eventId = EventId("e$seq"),
+            sessionId = SessionId("s"),
+            timestamp = Instant.fromEpochSeconds(0),
+            schemaVersion = 1,
+            causationId = null,
+            correlationId = null,
+        ),
+        sequence = seq,
+        sessionSequence = seq,
+        payload = payload,
+    )
+
+    @Test
+    fun `latest summary event wins and records stable coverage`() {
+        // Replaying the same events must yield the same state — coverage is keyed on the stable
+        // event sequence, so no live LLM call is needed to rebuild (invariant #8).
+        var state = ContextState()
+        state = reducer.reduce(state, stored(
+            ContextSummarizedEvent(SessionId("s"), 10L, ArtifactId("a1"), tier = 2, entriesOmittedCount = 5), 1L))
+        state = reducer.reduce(state, stored(
+            ContextSummarizedEvent(SessionId("s"), 25L, ArtifactId("a2"), tier = 3, entriesOmittedCount = 8), 2L))
+
+        assertEquals(25L, state.summarizedThroughSequence)
+        assertEquals(ArtifactId("a2"), state.summaryArtifactId)
+        assertEquals(3, state.summaryTier)
+    }
+}