Fix all v4 audit findings (22 items)

HIGH: ISO writer multi-extent for >4GB, end-to-end MKV mux test
MEDIUM: AACS cvalue bounds, UV offset, macOS discovery, VC-1 resolution
  from sequence header, HEVC profile flags from SPS, ISO CRC + reserve AVDP
LOW: PS AC3 sub-header, CSS crack first-match break, TrackUID unique,
  AC3 no-sync empty return, --all for iso://, --min warning, dead code removed

320 tests, all passing.

src/mux/codec/ac3.rs

@@ -81,16 +81,7 @@ impl CodecParser for Ac3Parser {
             }
         }
 
-        // If we found no syncword at all, emit the whole PES as a frame
-        // (backwards-compatible with old behaviour).
-        if frames.is_empty() {
-            frames.push(Frame {
-                pts_ns,
-                keyframe: true,
-                data: data.to_vec(),
-            });
-        }
-
+        // If we found no syncword at all, return empty — the data is not valid AC3.
         frames
     }