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/src/fec/gf16.rs

@@ -12,7 +12,7 @@ impl ErasureCoder for Gf16Coder {
         FecScheme::Gf16
     }
 
-    fn encode(&self, data: &[Vec<u8>], recovery_count: usize) -> Result<Vec<Vec<u8>>, FecError> {
+    fn encode(&self, data: &[&[u8]], recovery_count: usize) -> Result<Vec<Vec<u8>>, FecError> {
         if recovery_count == 0 {
             return Ok(Vec::new());
         }