feat(core): zero-copy pooled reassembly — shards land at their final AU offset

Rewrite the client Reassembler around one whole-frame buffer per frame:
frame_bytes rides in every header and packetize geometry is
deterministic (every non-final block is exactly max_data_per_block data
shards), so a data shard's final AU offset is computable on arrival —
copy it there once, straight from the decrypt ring. New
ErasureCoder::reconstruct_into decodes ONLY the missing shards directly
into the frame buffer's holes (gf16 native; gf8 legacy shim); received
recovery shards ride pooled shard-sized buffers. The completed buffer
IS Frame::data.

Deletes the per-shard to_vec + per-block concat + final AU concat
(~178k allocs and a double copy of every byte per second at 2 Gbps —
the pump wall the 2026-07-14 sweeps measured at 98.9% of an M3 Ultra
core). Reassembly now costs ~0.4 µs/packet in-stream.

The eager buffer changes the hostile-header exposure, so two new
firewalls: derived-geometry validation (a header lying about its
data_shards/block_count vs its own frame_bytes is dropped before it can
scribble across another shard's range) and an in-flight allocation
budget (IN_FLIGHT_BUF_FACTOR × max_frame_bytes) so a window of tiny
first-shards can't commit gigabytes.

Behavior parity pinned by the existing suite (all green unchanged) plus
new end-to-end roundtrips through the real Packetizer (multi-block +
partial tail, loss within budget, reversed delivery, duplicates, empty
frame, unrecoverable block ages out, budget enforcement). loss-harness
recovery curve identical; pipeline bench: gf8/1MB +42%, gf16 neutral
(host-encode dominated). Known pre-existing quirk kept as-is: reversed
delivery reconstructs early (data+recovery ≥ k) and counts late-not-lost
shards into fec_recovered_shards.

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

crates/punktfunk-core/src/fec/gf16.rs

@@ -2,7 +2,9 @@
 //! shards/block — this is what removes the GameStream 255-shard / ~1 Gbps wall.
 //! Shard length must be even.
 
-use super::{validate_block_shape, validate_encode_shape, ErasureCoder, FecError};
+use super::{
+    validate_block_shape, validate_encode_shape, validate_into_shape, ErasureCoder, FecError,
+};
 use crate::config::FecScheme;
 
 pub struct Gf16Coder;
@@ -81,4 +83,46 @@ impl ErasureCoder for Gf16Coder {
         }
         Ok(out)
     }
+
+    fn reconstruct_into(
+        &self,
+        recovery_count: usize,
+        data: &mut [&mut [u8]],
+        have: &[bool],
+        recovery: &[(usize, &[u8])],
+    ) -> Result<(), FecError> {
+        validate_into_shape(data, have, recovery, recovery_count)?;
+        if have.iter().all(|h| *h) {
+            return Ok(()); // nothing missing — no codec work, no copies
+        }
+        if data[0].len() % 2 != 0 {
+            return Err(FecError::Config("GF(2^16) shard length must be even"));
+        }
+        let data_count = data.len();
+        // Present originals as indexed refs (shared reborrows of the caller's slots); the decoder
+        // returns the restored shards owned, so the borrows end before the write-back below.
+        let original_in: Vec<(usize, &[u8])> = data
+            .iter()
+            .zip(have)
+            .enumerate()
+            .filter(|(_, (_, &h))| h)
+            .map(|(i, (s, _))| (i, &**s))
+            .collect();
+        let restored = reed_solomon_simd::decode(
+            data_count,
+            recovery_count,
+            original_in,
+            recovery.iter().copied(),
+        )
+        .map_err(|_| FecError::Backend("gf16 decode"))?;
+        for (i, h) in have.iter().enumerate() {
+            if !*h {
+                let shard = restored
+                    .get(&i)
+                    .ok_or(FecError::Backend("gf16 decode left an original missing"))?;
+                data[i].copy_from_slice(shard);
+            }
+        }
+        Ok(())
+    }
 }