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/ac3.rs

@@ -0,0 +1,37 @@
+//! AC3 (Dolby Digital) / EAC3 (Dolby Digital Plus) frame parser.
+//!
+//! AC3 frames are self-contained and always start with syncword 0x0B77.
+//! Each PES packet typically contains exactly one AC3 frame.
+//! All AC3 frames are effectively keyframes (no inter-frame dependencies).
+
+use super::{CodecParser, Frame, PesPacket, pts_to_ns};
+
+pub struct Ac3Parser;
+
+impl Ac3Parser {
+    pub fn new() -> Self {
+        Self
+    }
+}
+
+impl CodecParser for Ac3Parser {
+    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);
+
+        // AC3: each PES = one frame, always a keyframe
+        vec![Frame {
+            pts_ns,
+            keyframe: true,
+            data: pes.data.clone(),
+        }]
+    }
+
+    fn codec_private(&self) -> Option<Vec<u8>> {
+        // AC3 doesn't need codecPrivate in MKV
+        None
+    }
+}