Add 113 tests, update CI to checkout@v5, add FEATURES.md

Test suite: 64 → 177 tests
- MPLS parser: 6 tests (synthetic binary, streams, errors)
- CLPI parser: 6 tests (EP map, PTS/SPN math, errors)
- H.264: 12 tests (NAL parsing, SPS/PPS, keyframes)
- HEVC: 13 tests (VPS/SPS/PPS, IRAP range, codec private)
- AC3: 12 tests (syncword, frame extraction)
- VC1: 15 tests (BITMAPINFOHEADER, start codes)
- DTS: 5, TrueHD: 4, PGS: 4 tests
- EBML: 6 tests (size/ID/string/float roundtrips)
- UDF: 10 tests (MockSectorReader, filesystem parsing, error paths)
- Disc: 8 tests (scan_image, DiscTitle helpers)
- Streams: 5 new (meta roundtrip, MkvStream)
- NullStream: 4, StdioStream: 2, IsoSectorReader: 2

CI: actions/checkout@v4 → v5 (all workflows)
FEATURES.md: created for v0.7.1

src/mux/stdio.rs

@@ -75,3 +75,26 @@ impl Write for StdioStream {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::io::{Read, Write};
+
+    #[test]
+    fn stdio_output_write_errors_on_read() {
+        let mut stream = StdioStream::output();
+        let mut buf = [0u8; 10];
+        let err = stream.read(&mut buf).unwrap_err();
+        assert_eq!(err.kind(), io::ErrorKind::Unsupported);
+        assert!(err.to_string().contains("cannot read"), "got: {}", err);
+    }
+
+    #[test]
+    fn stdio_input_read_errors_on_write() {
+        let mut stream = StdioStream::input();
+        let err = stream.write(&[0u8; 10]).unwrap_err();
+        assert_eq!(err.kind(), io::ErrorKind::Unsupported);
+        assert!(err.to_string().contains("cannot write"), "got: {}", err);
+    }
+}