Fix M2TS/MKV roundtrip: codec_private in FMKV header, Annex B conversion

Bug 1: M2TS roundtrip dropped frames — TsMuxer converts length-prefixed
NALs to Annex B, prepends VPS/SPS/PPS from HEVCDecoderConfigurationRecord.

Bug 2: MKV remux lost codec_private — MkvStream.codec_private() now returns
data from EBML header.

FMKV header carries codec_private (base64) per video stream for lossless
M2TS roundtrip.

src/mux/resolve.rs

@@ -384,7 +384,7 @@ pub fn output(
         }
         StreamUrl::M2ts { ref path } => {
             validate_file_path(path, "m2ts")?;
-            Ok(Box::new(super::pesout::M2tsOutputStream::create(&path.to_string_lossy(), title)?))
+            Ok(Box::new(super::pesout::M2tsOutputStream::create(&path.to_string_lossy(), title, codec_privates)?))
         }
         StreamUrl::Network { ref addr } => {
             validate_network_addr(addr)?;