Add IOStream trait and stream-based I/O architecture

Introduce IOStream trait for uniform read/write across disc, file,
network, and null streams. Rename Title→DiscTitle, add stream URL
resolver, split old stream.rs into focused modules (m2ts, mkvstream,
network, disc, null, resolve, meta).

src/mux/codec/hevc.rs

@@ -34,7 +34,8 @@ impl CodecParser for HevcParser {
             return Vec::new();
         }
 
-        let pts_ns = pes.pts.map(pts_to_ns).unwrap_or(0);
+        // Use DTS when available (monotonic for B-frame content), fall back to PTS
+        let pts_ns = pes.dts.or(pes.pts).map(pts_to_ns).unwrap_or(0);
         let data = &pes.data;
         let mut keyframe = false;