feat(portal): async encode pipeline - decouple capture from encoding

Split synchronous encode pipeline so sws_scale + libx264 runs on a
dedicated thread, leaving only VAAPI import + GPU scale + GPU→CPU
transfer on the main capture thread.

Problem: encode_p95 occasionally hit 74ms, blocking the entire capture
pipeline and causing capture_gap_max=356ms stutter.

Solution:
- avhw.rs: Split SwEncState into SwEncImport (main thread: VAAPI import,
  filter_graph scale, GPU→CPU transfer) and SwEncEncode (encode thread:
  sws_scale NV12→YUV420P, libx264 encode). New CpuNv12Frame struct
  carries owned pixel data across threads via crossbeam channel.
  SwEncState wraps both for backward compat (MP4/sync path untouched).
- state_portal.rs: WebRTC portal path spawns 'wl-webrtc-encode' thread
  with bounded(2) input channel (drop-newest backpressure) and separate
  timing channel. Graceful shutdown: drop webrtc_rx → drop input_tx →
  join encode thread → flush sync encoder.
- stats.rs: Add record_import() + record_encode_thread() for async timing.

Results: encode_p95 stable at 2.9-4.2ms (was 11-74ms), capture_fps
stable 59-60fps, cap_gap_p95 17-19ms. Remaining capture stalls traced
to PipeWire compositor frame delivery (external, not our code).

src/main.rs

@@ -15,6 +15,7 @@ mod backend_detect; // 截屏后端自动检测（wlroots vs Portal/PipeWire）
 mod cap_portal; // XDG Portal 屏幕捕获
 mod cap_wlr_screencopy; // wlroots wlr-screencopy 截屏协议
 mod fps_limit; // 帧率限制器
+mod stats; // 管道性能统计（卡顿诊断）
 mod state; // wlr-screencopy 后端的主状态机
 mod state_portal; // Portal/PipeWire 后端的主状态机
 mod transform; // 图像变换（旋转/翻转）
@@ -43,18 +44,23 @@ fn main() -> Result<()> {
     // 解析命令行参数
     let args = Args::parse();
 
-    // 根据是否启用 verbose 模式设置日志级别
+    // 根据 verbose 模式或 RUST_LOG 环境变量设置日志级别
+    // 支持 RUST_LOG 粒度控制（如 RUST_LOG=wl_webrtc::webrtc=trace）
+    let env_filter = tracing_subscriber::EnvFilter::try_from_default_env()
+        .unwrap_or_else(|_| {
+            if args.verbose {
+                tracing_subscriber::EnvFilter::new("debug")
+            } else {
+                tracing_subscriber::EnvFilter::new("info")
+            }
+        });
     tracing_subscriber::fmt()
-        .with_max_level(if args.verbose {
-            tracing::Level::DEBUG
-        } else {
-            tracing::Level::INFO
-        })
+        .with_env_filter(env_filter)
         .with_writer(std::io::stderr)
         .init();
 
     tracing::info!("wl-webrtc starting");
-    tracing::debug!("Args: {:?}", args);
+    tracing::debug!("Args: output={:?} fps={} codec={} port={} verbose={}", args.output, args.fps, args.codec, args.port, args.verbose);
 
     // MVP 阶段仅支持 H.264 编码，不支持 HEVC
     if args.codec != "h264" {
@@ -250,7 +256,7 @@ fn run_wlr_screencopy(args: Args) -> Result<()> {
 
         // 状态机遇到致命错误时退出
         if state.errored {
-            tracing::error!("Fatal error in state machine, exiting");
+            tracing::error!("Fatal error in state machine (check preceding error logs), exiting");
             running = false;
         }
 
@@ -346,7 +352,7 @@ fn run_portal_pipewire(args: Args) -> Result<()> {
 
         // Portal 状态机遇到致命错误时退出
         if state.is_errored() {
-            tracing::error!("Fatal error in portal state machine, exiting");
+            tracing::error!("Fatal error in portal state machine (check preceding error logs), exiting");
             running = false;
         }
     }