Defer OCIO LUT processor generation in render worker and fix stale graph snapshots

- Override OCIOLutNode::Value() to ensure the processor is created/updated
  before the color transform job is emitted.
- In the worker process, mark the processor dirty on input/config changes
  instead of creating it synchronously during LoadGraph, which blocked the
  main process waiting for the graph load acknowledgement.
- Keep eager processor generation in the main GUI process so the viewer
  cache is invalidated immediately when the LUT file or direction changes.
- Mark the ProjectCopier's internal render-proxy project as modified when
  its update queue is processed, and reset the flag after RenderWorkerPool
  writes a new graph snapshot. This fixes the worker loading a stale graph
  snapshot after switching cube files or direction, which caused the old
  LUT effect to persist.

All existing tests pass.

app/render/renderworkerpool.cpp

@@ -572,6 +572,12 @@ bool RenderWorkerPool::PrepareJob(RenderTicketPtr ticket,
 				return false;
 			}
 			wrote_new_snapshot = true;
+			// For internal render-proxy copies, the snapshot now represents the
+			// serialized state, so reset the modified flag so the snapshot can be
+			// reused until the copy changes again.
+			if (project->property("_oak_render_proxy").toBool()) {
+				project->set_modified(false);
+			}
 			locker.relock();
 			graph_cache_.insert(project_uuid, {graph_path});
 		}