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/args.rs

@@ -50,4 +50,8 @@ pub struct Args {
     /// Force re-authorization dialog (ignore saved portal restore token)
     #[arg(long)]
     pub no_persist: bool,
+
+    /// Enable per-second pipeline statistics output for stutter diagnosis
+    #[arg(long)]
+    pub stats: bool,
 }