Add MKV muxer and event system

- Add event.rs: structured event system for progress reporting
- Add mux/: MKV muxer pipeline (EBML writer, TS demuxer, stream assembly)
  - Codec parsers: H.264, HEVC, AC-3, DTS, TrueHD, PGS, VC-1
  - Lookahead buffer for codec private data extraction
  - Direct m2ts-to-MKV streaming without intermediate files

src/mux/codec/pgs.rs

@@ -0,0 +1,25 @@
+//! HDMV PGS (Presentation Graphics Stream) subtitle parser.
+//!
+//! PGS segments: PCS, WDS, PDS, ODS, END.
+//! Each PES packet contains one or more segments.
+//! All segments are keyframes (no inter-segment dependencies).
+
+use super::{CodecParser, Frame, PesPacket, pts_to_ns};
+
+pub struct PgsParser;
+
+impl PgsParser {
+    pub fn new() -> Self { Self }
+}
+
+impl CodecParser for PgsParser {
+    fn parse(&mut self, pes: &PesPacket) -> Vec<Frame> {
+        if pes.data.is_empty() {
+            return Vec::new();
+        }
+        let pts_ns = pes.pts.map(pts_to_ns).unwrap_or(0);
+        vec![Frame { pts_ns, keyframe: true, data: pes.data.clone() }]
+    }
+
+    fn codec_private(&self) -> Option<Vec<u8>> { None }
+}