feat(stats): add --stats flag and PipelineStats windowed diagnostics

Add lightweight per-second pipeline statistics for stutter diagnosis:
- --stats CLI flag enables structured stats logging
- PipelineStats tracks capture/encode/send timing with p95/pmax
- FrameTimings records import/scale/transfer/sws/encode per-frame
- StatsSnapshot produces one structured log line per second

src/lib.rs

@@ -4,6 +4,7 @@ pub mod backend_detect;
 pub mod cap_portal;
 pub mod cap_wlr_screencopy;
 pub mod fps_limit;
+pub mod stats;
 pub mod state;
 pub mod state_portal;
 pub mod transform;