feat(gpu-decode): zero-copy hardware imports and the planar pipeline

Adds VAAPI DMA-BUF, D3D11VA shared-handle and VideoToolbox IOSurface
imports behind a tri-state outcome (imported / unsupported / failed),
planar textures with bounded residency and a CPU staging fallback, the
staged montage decode path, reference-counted decoder frames, VAAPI-first
device selection on Linux, and the host-GPU context plumbing used by the
app and worker. See docs/zh/plans/render-pipeline-threads.md (M5).

crates/oak-task/src/export.rs

@@ -295,6 +295,13 @@ impl ExportTask {
 					"Render frame readback for the encoder failed: {e:?}"
 				))
 			})?,
+			// Unresolved planar frames never reach an encoder; the
+			// decode path resolves them before delivery.
+			Texture::Planar(_) => {
+				return Err(Error::Failed(
+					"unresolved planar texture for the encoder".into(),
+				))
+			}
 		};
 		let frame = &frame;
 		let params = CommonVideoParams::new_basic(

crates/oak-task/src/nodeops.rs

@@ -1181,3 +1181,4 @@ pub fn add_track_command(project: ProjectRef, list: NodeId) -> UndoCommand {
 		kind: TrackType::Video,
 	})
 }
+ 		kind: TrackType::Video,