feat: GPU-downscale + software H.264 encode pipeline (WIP)

Add SwEncState in avhw.rs: GPU pipeline using scale_vaapi to downscale
4K BGRA -> 2K NV12 on AMD iGPU, then software encode with libopenh264.

- import_dma_buf_to_vaapi: av_hwframe_map based DMA-BUF import
- SwEncState: GPU filter graph (scale_vaapi) + NV12->YUV420P + libopenh264
- state_portal.rs: integrated SwEncState, auto DRM device detection
- vaapi_import_bench.rs: CPU vs GPU pipeline benchmark
- sw_encode_bench.rs: software encode benchmark

Benchmark results: GPU pipeline ~91 FPS theoretical (10.95ms/frame)
vs CPU pipeline ~33 FPS (30.21ms/frame).

Known issue: only 1 frame encoded in production recording,
diagnostic STATS logging added to debug frame flow.

src/backend_detect.rs

@@ -137,10 +137,7 @@ pub fn detect_backend(args: &Args) -> Result<CaptureBackend> {
             }
             other => {
                 // 未知后端名称，返回错误
-                anyhow::bail!(
-                    "Unknown backend '{}'. Use 'screencopy' or 'portal'.",
-                    other
-                );
+                anyhow::bail!("Unknown backend '{}'. Use 'screencopy' or 'portal'.", other);
             }
         };
     }