feat(host/windows): USO batched send for the GameStream video plane

The GameStream video sender did one send() syscall per packet on Windows
(the #[cfg(not(target_os="linux"))] sendmmsg_all fallback), capping
throughput at high packet rates. Wire it to UDP Send Offload (the Windows
analogue of Linux GSO) so each paced 16-packet burst goes out in one
WSASendMsg(UDP_SEND_MSG_SIZE) syscall instead of 16, preserving the
microburst pacing.

Expose a reusable punktfunk_core::transport::send_uso_all (Windows-only)
that reuses the proven native-plane USO primitive (send_one_uso + the uso
on/off latch + uso_unsupported), with the same uniform-size guard and
≤512-segment chunking as UdpTransport::send_gso. It returns how many leading
packets it sent via USO; the GameStream sendmmsg_all sends any remainder
(USO off via PUNKTFUNK_GSO=0, a size-mixed burst, or a frame's short final
packet) with per-packet send. On-wire packet boundaries are unchanged.

Resolves #4 in docs/apollo-comparison.md. Linux build unaffected;
punktfunk-core type-checks for x86_64-pc-windows-msvc. Host Windows compile
deferred to CI / dev box.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -5,6 +5,10 @@ mod loopback;
 mod udp;
 
 pub use loopback::{loopback_pair, LoopbackTransport};
+/// Windows-only: reusable USO (UDP Send Offload) batch send for callers that own their own connected
+/// socket (the GameStream video sender) rather than going through [`UdpTransport`].
+#[cfg(target_os = "windows")]
+pub use udp::send_uso_all;
 pub use udp::{spawn_data_punch, UdpTransport, PUNCH_MAGIC};
 
 /// A datagram transport. `recv` is non-blocking: it returns `Ok(None)` when no packet