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-host/src/gamestream/video.rs

@@ -149,7 +149,8 @@ impl VideoPacketizer {
             };
             let wire_pct = if m > 0 { (100 * m) / k } else { 0 };
             let parity = if m > 0 {
-                Gf8Coder.encode(&shards, m).unwrap_or_default()
+                let refs: Vec<&[u8]> = shards.iter().map(|s| s.as_slice()).collect();
+                Gf8Coder.encode(&refs, m).unwrap_or_default()
             } else {
                 Vec::new()
             };