perf(portal): achieve 58-60fps PipeWire screen capture

- Force PipeWire quantum=512 via NODE_FORCE_QUANTUM (48000/512=93Hz scheduling)
- Switch to libx264 ultrafast/zerolatency with 6 threads
- Use two-phase poll_and_encode: blocking recv_timeout for first frame,
  non-blocking try_recv drain for subsequent frames
- Remove fps_limit from portal path (PW already rate-limits via quantum/KWin;
  fps_limit's min_interval was silently dropping ~10% of valid frames)
- Remove diagnostic instrumentation (TIMING/PIPEWIRE logs, timing fields,
  pw_stats counters)
- Add lightweight production stats: per-10s fps log + shutdown summary
- Prefer libx264 over libopenh264 (better quality at same speed)

src/avhw.rs

@@ -996,10 +996,10 @@ fn create_software_h264_muxer(
     ff::format::context::Output,
 )> {
     let output_cstr = CString::new(output_path.to_str().unwrap())?;
-    let codec = ff::encoder::find_by_name("libopenh264")
-        .or_else(|| ff::encoder::find_by_name("libx264"))
+    let codec = ff::encoder::find_by_name("libx264")
+        .or_else(|| ff::encoder::find_by_name("libopenh264"))
         .ok_or_else(|| {
-            anyhow::anyhow!("No H.264 software encoder found (tried libopenh264, libx264)")
+            anyhow::anyhow!("No H.264 software encoder found (tried libx264, libopenh264)")
         })?;
     let codec_name = codec.name().to_string();
 
@@ -1024,11 +1024,14 @@ fn create_software_h264_muxer(
         // SAFETY: priv_data belongs to the unopened encoder; strings live for each call.
         unsafe {
             let key = CString::new("preset").unwrap();
-            let val = CString::new("veryfast").unwrap();
+            let val = CString::new("ultrafast").unwrap();
             ffi::av_opt_set((*enc.as_mut_ptr()).priv_data, key.as_ptr(), val.as_ptr(), 0);
             let key = CString::new("tune").unwrap();
             let val = CString::new("zerolatency").unwrap();
             ffi::av_opt_set((*enc.as_mut_ptr()).priv_data, key.as_ptr(), val.as_ptr(), 0);
+            let key = CString::new("threads").unwrap();
+            let val = CString::new("6").unwrap();
+            ffi::av_opt_set((*enc.as_mut_ptr()).priv_data, key.as_ptr(), val.as_ptr(), 0);
         }
     }