libfreemkv 0.31.2: comprehensive spec-grounded test suite (~950 tests)

Test-hardening release, no runtime changes. Adds spec-grounded unit tests
across the silent-corruption surfaces — UDF/MPLS/CLPI/IFO parsing, BD/DVD
title + extent assembly, AACS/CSS key handling, TS/PS demux + codec parsers,
MKV/EBML container output, the mux pipeline, sector prefetch + decrypt
decorator, drive/SCSI sense decoding, label extraction, and core I/O. Each
test is grounded in the format spec or real on-disc behavior and verified to
fail under a targeted source mutation. No behavior changed.

src/mux/null.rs

@@ -66,4 +66,51 @@ mod tests {
         let err = Stream::read(&mut sink).expect_err("read on a sink must error");
         assert_eq!(err.kind(), io::ErrorKind::Unsupported);
     }
+
+    /// finish() must be idempotent and safe to call repeatedly — a benchmark
+    /// driver may finish more than once. Each must be Ok(()), and writes
+    /// after finish must still succeed (NullStream has no terminal state).
+    #[test]
+    fn finish_is_idempotent_and_write_after_finish_ok() {
+        let title = DiscTitle::empty();
+        let mut sink = NullStream::new(&title);
+        sink.finish().unwrap();
+        sink.finish().unwrap();
+        let frame = crate::pes::PesFrame {
+            track: 3,
+            pts: 42,
+            keyframe: false,
+            data: vec![0xFF; 4096],
+            duration_ns: Some(1000),
+        };
+        // Discard-sink contract: write always returns Ok regardless of frame
+        // size, track index, or post-finish state.
+        sink.write(&frame).unwrap();
+    }
+
+    /// info() must return the title the sink was constructed with, unchanged
+    /// — the Stream trait contract requires info() be stable and reflect the
+    /// supplied metadata (the muxer reads stream layout from it).
+    #[test]
+    fn info_reflects_constructed_title() {
+        let mut title = DiscTitle::empty();
+        title.playlist = "BenchTitle".into();
+        title.playlist_id = 7;
+        let sink = NullStream::new(&title);
+        assert_eq!(sink.info().playlist, "BenchTitle");
+        assert_eq!(sink.info().playlist_id, 7);
+    }
+
+    /// The write-only read() guard must hold on EVERY call, not just the
+    /// first — a caller that retries read() after the initial error must
+    /// keep getting StreamWriteOnly, never a stale Ok(None).
+    #[test]
+    fn read_stays_write_only_across_repeated_calls() {
+        let title = DiscTitle::empty();
+        let mut sink = NullStream::new(&title);
+        for _ in 0..3 {
+            let err = Stream::read(&mut sink).expect_err("read on a sink must always error");
+            assert_eq!(err.kind(), io::ErrorKind::Unsupported);
+        }
+    }
 }