fix(validation): resolve F-007 artifact validator CAS-hash conflation

ArtifactPayloadValidator passed the logical slot name into the
content-hash-keyed CAS store, crashing every workflow with a typed
produces slot. Validator now reads payloads from a content map threaded
through ValidationContext (populated from the orchestrator's per-session
artifact cache across all stages) and no longer depends on ArtifactStore.

Also records the slot->CAS-hash mapping as a new ArtifactContentStoredEvent
(registered for serialization), emitted at both CAS write sites, so
artifact content is recoverable from CAS by hash after a cold start.

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

@@ -30,3 +30,12 @@ data class ArtifactCreatedEvent(
     val stageId: StageId,
     val schemaVersion: Int,
 ) : EventPayload
+
+@Serializable
+@SerialName("ArtifactContentStored")
+data class ArtifactContentStoredEvent(
+    val artifactId: ArtifactId,
+    val contentHash: ArtifactId, // CAS content hash returned by ArtifactStore.put(...)
+    val sessionId: SessionId,
+    val stageId: StageId,
+) : EventPayload

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

@@ -4,6 +4,7 @@ import com.correx.core.events.events.ApprovalDecisionResolvedEvent
 import com.correx.core.events.events.ApprovalGrantCreatedEvent
 import com.correx.core.events.events.ApprovalGrantExpiredEvent
 import com.correx.core.events.events.ApprovalRequestedEvent
+import com.correx.core.events.events.ArtifactContentStoredEvent
 import com.correx.core.events.events.ArtifactCreatedEvent
 import com.correx.core.events.events.ArtifactValidatedEvent
 import com.correx.core.events.events.ArtifactValidatingEvent
@@ -70,6 +71,7 @@ val eventModule = SerializersModule {
         subclass(ApprovalDecisionResolvedEvent::class)
         subclass(ApprovalGrantCreatedEvent::class)
         subclass(ApprovalGrantExpiredEvent::class)
+        subclass(ArtifactContentStoredEvent::class)
         subclass(ArtifactCreatedEvent::class)
         subclass(ArtifactValidatingEvent::class)
         subclass(ArtifactValidatedEvent::class)