perf(core): ref-based FEC encode — packetize shards reference the frame in place

Stage A of the zero-copy host packetize path (networking-audit deferred
plan §1): ErasureCoder::encode now takes &[&[u8]], so Packetizer::packetize
builds each block's data shards as slices straight into the frame buffer
instead of allocating + copying a Vec per data shard. Only the frame's
final (possibly partial) shard is staged in a reusable zero-padded scratch;
blocks are consecutive shard ranges, so every other shard is a full
payload-sized slice.

- gf8: encode_sep() over the same Cauchy codec — parity byte-identical to
  nanors/Moonlight (nanors_exact_parity_vectors unchanged and green)
- gf16: reed_solomon_simd::encode is already generic over AsRef<[u8]>
- loss-harness sweep: recovery rates identical before/after
- bench pipeline (end-to-end, host+client): gf8/64K -3.0%, gf16/64K -2.2%,
  gf16/1M -3.4%, gf8/1M -0.7%

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/punktfunk-core/tests/loopback.rs

@@ -205,7 +205,8 @@ proptest! {
             let data: Vec<Vec<u8>> = (0..k)
                 .map(|i| (0..shard_len).map(|b| (i ^ b).wrapping_add(seed as usize) as u8).collect())
                 .collect();
-            let recovery = coder.encode(&data, m).unwrap();
+            let refs: Vec<&[u8]> = data.iter().map(|s| s.as_slice()).collect();
+            let recovery = coder.encode(&refs, m).unwrap();
 
             let mut received: Vec<Option<Vec<u8>>> =
                 data.iter().cloned().map(Some).chain(recovery.into_iter().map(Some)).collect();