feat: inject recalled L3 memory into router context with budget

Record L3 retrieval as an event carrying hit text (invariant #9), then
rebuild router state and inject recalled memories as a SYSTEM L3 layer in
the context pack. Apply token budget: protected frames (L0 immutable +
current user turn) are never dropped; honest budgetUsed is reported and
'BudgetExceeded' is flagged in appliedStrategies when they overflow. L1/L2
fit newest-first so oldest entries drop first. Emit ContextTruncatedEvent
when entries are dropped. L3MemoryRetrievedEvent is emitted on every CHAT
turn (empty hits reset recalled memory).

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

@@ -37,3 +37,13 @@ data class L3RetrievedHit(
     val text: String,
     val score: Float,
 )
+
+@Serializable
+@SerialName("ContextTruncated")
+data class ContextTruncatedEvent(
+    val sessionId: SessionId,
+    val turnId: String,
+    val entriesDropped: Int,
+    val truncatedLayers: List<String>,
+    val timestampMs: Long,
+) : EventPayload

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

@@ -9,6 +9,7 @@ import com.correx.core.events.events.ArtifactValidatedEvent
 import com.correx.core.events.events.ArtifactValidatingEvent
 import com.correx.core.events.events.ChatSessionStartedEvent
 import com.correx.core.events.events.ChatTurnEvent
+import com.correx.core.events.events.ContextTruncatedEvent
 import com.correx.core.events.events.EventPayload
 import com.correx.core.events.events.L3MemoryRetrievedEvent
 import com.correx.core.events.events.InferenceCompletedEvent
@@ -75,6 +76,7 @@ val eventModule = SerializersModule {
         subclass(ChatSessionStartedEvent::class)
         subclass(ChatTurnEvent::class)
         subclass(L3MemoryRetrievedEvent::class)
+        subclass(ContextTruncatedEvent::class)
     }
 }