Drop the redundant complete-block short circuit from FEC recovery

A block with every data shard present passes through the code as a no-op
and yields no recovered records, so the early return in both recovery
functions did nothing the code did not already do. Mutation showed both
survive removal. The audio payload-length rule is now pinned by its
message, since the codec's own shard-size error would otherwise mask its
removal.

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

packages/cultnet-rs/src/media_fec.rs

@@ -450,9 +450,6 @@ pub fn recover_video_block(
     }
 
     let missing: Vec<usize> = (0..k).filter(|&slot| shards[slot].is_none()).collect();
-    if missing.is_empty() {
-        return Ok(Vec::new());
-    }
     let recovered = recover_data_shards(k, shards)?;
     Ok(missing
         .into_iter()
@@ -617,9 +614,6 @@ pub fn recover_audio_block(
     }
 
     let missing: Vec<usize> = (0..k).filter(|&slot| shards[slot].is_none()).collect();
-    if missing.is_empty() {
-        return Ok(Vec::new());
-    }
     let recovered = recover_data_shards(k, shards)?;
     Ok(missing
         .into_iter()

packages/cultnet-rs/tests/media_fec.rs

@@ -592,7 +592,8 @@ fn an_audio_block_is_four_contiguous_equal_length_packets() {
     assert!(protect_audio_block(&packets, &STANDARD).is_err(), "a gap in presentation time");
     let mut packets = audio_block(1, 20);
     packets[3].payload.push(0);
-    assert!(protect_audio_block(&packets, &STANDARD).is_err(), "unequal payload lengths");
+    let error = protect_audio_block(&packets, &STANDARD).unwrap_err();
+    assert!(error.to_string().contains("payload length"), "unequal payload lengths: {error}");
     let mut packets = audio_block(1, 20);
     packets[0].codec = "pcm".to_string();
     assert!(protect_audio_block(&packets, &STANDARD).is_err(), "mixed codecs");