fix(kernel): llm-emitted artifacts require stored content — close phantom path

emitLlmArtifacts previously emitted ArtifactCreated/Validating/Validated for every
llm-emitted slot unconditionally, fabricating "validated" artifacts when the model
produced no content. Gate now mirrors the F-015 file_written check: slots with null
or blank cached content are skipped with a warn log. Regression test added in
NeedsArtifactInjectionTest verifies no artifact events appear for a blank-response stage.

core/kernel/src/main/kotlin/com/correx/core/kernel/orchestration/SessionOrchestrator.kt

@@ -1158,6 +1158,17 @@ abstract class SessionOrchestrator(
         stageConfig.produces
             .filter { it.kind.llmEmitted }
             .forEach { slot ->
+                // Same rule as the file_written gate (F-015): no stored content means the
+                // model never produced this artifact — do not fabricate a validated one.
+                val cached = artifactContentCache["${sessionId.value}:${slot.name.value}"]
+                if (cached.isNullOrBlank()) {
+                    log.warn(
+                        "[Orchestrator] stage={} slot={}: no content stored — skipping artifact emission",
+                        stageId.value,
+                        slot.name.value,
+                    )
+                    return@forEach
+                }
                 emit(sessionId, ArtifactCreatedEvent(slot.name, sessionId, stageId, schemaVersion = 1))
                 emit(sessionId, ArtifactValidatingEvent(slot.name, sessionId, stageId))
                 emit(sessionId, ArtifactValidatedEvent(slot.name, sessionId, stageId))