feat(core): scale the receive path to the new multi-Gbps ceiling

- REPLAY_WINDOW 32768 -> 131072: the anti-replay bitmap covered the
  120 ms loss window only to ~2 Gbps; the client now delivers ~4.8 Gbps
  wire, where a late-but-valid Wi-Fi-retried datagram would have been
  dropped as 'older than the window' — false loss. 16 KiB/session
  covers ~12 Gbps.
- RECV_BATCH 32 -> 128: syscall rate stays ~3.4k/s at 430k pkt/s and
  each pump iteration drains the kernel buffer deeper (ring 64->256 KB,
  client sessions only). flush_backlog's iteration cap rescaled to keep
  its ~190 MB guard equivalent.
- PUNKTFUNK_GSO gate is now value-aware: '=0' used to ENABLE GSO on
  Linux (presence check) while disabling Windows USO. GSO stays OPT-IN,
  deliberately: A/B'd twice today — it cuts send-thread CPU ~30% but
  its 16-packet line-rate trains cost delivered throughput on a
  constrained fabric (2.5GbE-hop pair: peak 2453 -> 1908 Mbps and 0.4%
  loss at a rate sendmmsg carries clean). Flipping the default belongs
  with pace-aware chunk spacing (plan Phase 1.2/1.3). docs-site row
  corrected to match.

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

crates/punktfunk-core/src/transport/mod.rs

@@ -46,7 +46,8 @@ pub trait Transport: Send + Sync {
     /// ~1 GSO skb per ≤64 segments instead of one skb per packet. This is the multi-Gbps lever —
     /// research shows ~2.4× throughput at equal CPU and ~40× fewer syscalls, and that `sendmmsg`
     /// batching alone is insufficient (it still builds one skb per datagram). The
-    /// [`UdpTransport`](super::UdpTransport) Linux override implements it (opt-in via `PUNKTFUNK_GSO`,
+    /// [`UdpTransport`](super::UdpTransport) Linux override implements it (opt-in via
+    /// `PUNKTFUNK_GSO=1` pending pace-aware chunk spacing — see the `gso` module doc — with
     /// auto-fallback on any GSO error); the default just delegates to [`send_batch`](Self::send_batch),
     /// correct for loopback and non-Linux. Same lossy, FEC-protected short-count contract as `send_batch`.
     fn send_gso(&self, packets: &[&[u8]]) -> std::io::Result<usize> {