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/bin/vaapi_import_bench.rs

@@ -186,10 +186,10 @@ fn drain_encoder(
 
 fn create_software_encoder(output_path: &Path, width: u32, height: u32) -> Result<SoftwareEncoder> {
     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();