Close the media FEC test gaps found before mutation

Pins the block-boundary chunk and last-chunk length refusals in video
recovery, the audio recovery refusals (packets one either side of the block,
another session, wrong length, parity from different blocks), the remaining
audio block identity rules (session, stream, timebase, duration), and the
data-parity alternation inside a block.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/cultnet-rs/tests/media_fec.rs

@@ -281,7 +281,8 @@ fn recovery_refuses_records_that_are_not_one_block() {
     assert!(matches!(recover_video_block(&mixed, &[]), Err(MediaFecError::Invalid(_))));
 
     // A chunk from block one handed to block zero's parity.
-    let stray = data.iter().find(|chunk| chunk.chunk_index == 20).unwrap().clone();
+    // The first chunk of block one, exactly one past block zero's last.
+    let stray = data.iter().find(|chunk| chunk.chunk_index == 16).unwrap().clone();
     assert!(matches!(recover_video_block(&block_zero, &[stray]), Err(MediaFecError::Invalid(_))));
 
     // A chunk whose length disagrees with the block's.
@@ -293,6 +294,59 @@ fn recovery_refuses_records_that_are_not_one_block() {
     ));
 
     assert!(matches!(recover_video_block(&[], &data), Err(MediaFecError::Invalid(_))));
+
+    // Another frame's chunk.
+    let mut other_frame = data.iter().find(|chunk| chunk.chunk_index == 3).unwrap().clone();
+    other_frame.frame_id += 1;
+    assert!(matches!(recover_video_block(&block_zero, &[other_frame]), Err(MediaFecError::Invalid(_))));
+}
+
+#[test]
+fn a_final_chunk_of_the_wrong_length_is_refused() {
+    let original = frame(8, 5, 100, 7);
+    let (data, parity) = split(protect_video_frame(&original, &STANDARD).unwrap());
+    let mut last = data.iter().find(|chunk| chunk.chunk_index == 4).unwrap().clone();
+    last.payload.push(0);
+    assert!(matches!(recover_video_block(&parity, &[last]), Err(MediaFecError::Invalid(_))));
+}
+
+#[test]
+fn within_a_block_data_and_parity_alternate() {
+    let records = protect_video_frame(&frame(9, 4, 20, 20), &STANDARD).unwrap();
+    let kinds: String = records
+        .iter()
+        .map(|record| match record {
+            GameCultMediaWireRecord::Video(_) => 'D',
+            GameCultMediaWireRecord::VideoParity(_) => 'P',
+            _ => '?',
+        })
+        .collect();
+    assert_eq!(kinds, "DPDPDD", "k = 4, m = 2: the longer list finishes the lane");
+}
+
+#[test]
+fn audio_recovery_refuses_packets_that_are_not_from_the_block() {
+    let original = audio_block(100, 20);
+    let parity = protect_audio_block(&original, &STANDARD).unwrap();
+    let next_block = audio_block(104, 20);
+    // One past the block's last packet, and one before its first.
+    for stray in [next_block[0].clone(), audio_block(96, 20)[3].clone()] {
+        assert!(matches!(
+            recover_audio_block(&parity, &[stray]),
+            Err(MediaFecError::Invalid(_))
+        ));
+    }
+    let mut other_session = original[0].clone();
+    other_session.session_id = "session-2".to_string();
+    assert!(matches!(recover_audio_block(&parity, &[other_session]), Err(MediaFecError::Invalid(_))));
+    let mut wrong_length = original[0].clone();
+    wrong_length.payload.push(0);
+    assert!(matches!(recover_audio_block(&parity, &[wrong_length]), Err(MediaFecError::Invalid(_))));
+    assert!(matches!(recover_audio_block(&[], &original), Err(MediaFecError::Invalid(_))));
+
+    let mut mixed = parity.clone();
+    mixed[1].base_packet_id += 1;
+    assert!(matches!(recover_audio_block(&mixed, &original[..3]), Err(MediaFecError::Invalid(_))));
 }
 
 #[test]
@@ -542,6 +596,18 @@ fn an_audio_block_is_four_contiguous_equal_length_packets() {
     let mut packets = audio_block(1, 20);
     packets[0].codec = "pcm".to_string();
     assert!(protect_audio_block(&packets, &STANDARD).is_err(), "mixed codecs");
+    let mut packets = audio_block(1, 20);
+    packets[2].session_id = "session-2".to_string();
+    assert!(protect_audio_block(&packets, &STANDARD).is_err(), "mixed sessions");
+    let mut packets = audio_block(1, 20);
+    packets[2].stream_id = "other".to_string();
+    assert!(protect_audio_block(&packets, &STANDARD).is_err(), "mixed streams");
+    let mut packets = audio_block(1, 20);
+    packets[1].timebase_den = 44_100;
+    assert!(protect_audio_block(&packets, &STANDARD).is_err(), "mixed timebases");
+    let mut packets = audio_block(1, 20);
+    packets[3].duration_ticks = 480;
+    assert!(protect_audio_block(&packets, &STANDARD).is_err(), "mixed durations");
 }
 
 #[test]