feat(host/windows): UDP send offload + NVENC 2-way split-encode (1 Gbps+ / 5K@240)

The Windows host couldn't sustain high-throughput / high-fps streams — two gaps vs the Linux host,
both found via live RTX 4090 measurement (PERF timing + nvidia-smi per-engine attribution):

- UDP Send Offload (USO). punktfunk-core's UdpTransport sent one packet per `send` syscall on
  Windows (send_batch/send_gso were Linux-only), capping throughput at high packet rates. Add a
  Windows `send_gso` override using `WSASendMsg` + `UDP_SEND_MSG_SIZE` (the Windows analogue of
  Linux UDP GSO) via windows-sys — one syscall segments a coalesced <=512-segment super-buffer to
  the connected peer. On by default with auto-fallback (PUNKTFUNK_GSO=0 disables, error latches
  off); plugs into the existing paced send path. SO_SNDBUF (32MB) was already cross-platform.

- NVENC 2-way split-frame encoding. A single Ada NVENC session tops out ~0.8 Gpix/s, so 5K@240
  (1.77 Gpix/s) took ~8 ms/frame -> a ~125 fps ceiling at high motion (the in-game stutter). Set
  NV_ENC_INITIALIZE_PARAMS.splitEncodeMode = TWO_FORCED above ~1 Gpix/s (matching the Linux
  libavcodec split_encode_mode path) to use both 4090 encoders — measured ~8 ms -> ~4 ms/frame at
  throughput. Env override PUNKTFUNK_SPLIT_ENCODE; init-failure fallback disables it (e.g. H264).

Windows-only paths; Linux/macOS unaffected. Builds clean on x86_64-pc-windows-msvc.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/punktfunk-core/Cargo.toml

@@ -58,6 +58,15 @@ tokio = { version = "1", optional = true, features = ["rt-multi-thread", "net",
 [target.'cfg(unix)'.dependencies]
 libc = "0.2"
 
+# Windows UDP Send Offload (USO): `WSASendMsg` + `UDP_SEND_MSG_SIZE` is the Windows analogue of
+# Linux UDP GSO — the 1 Gbps+ send lever (the host otherwise sends one packet per `send` syscall,
+# which caps throughput at high packet rates). See transport/udp.rs.
+[target.'cfg(windows)'.dependencies]
+# windows-sys (raw FFI, the quinn-udp choice): the high-level `windows` crate doesn't bind the
+# `WSASendMsg` extension function. WinSock feature gives WSASendMsg + WSAMSG/WSABUF/CMSGHDR.
+# Win32_System_IO too: WSASendMsg's signature references OVERLAPPED, so it's gated on that feature.
+windows-sys = { version = "0.59", features = ["Win32_Networking_WinSock", "Win32_System_IO"] }
+
 [dev-dependencies]
 proptest = "1"
 # Tier-1 microbenchmarks (benches/pipeline.rs). default-features off → no plotters/HTML (headless

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

@@ -131,6 +131,118 @@ fn send_one_gso(fd: libc::c_int, buf: &[u8], gso_size: u16) -> std::io::Result<(
     Ok(())
 }
 
+/// Windows UDP Send Offload (USO) enable state (process-wide). The Windows analogue of Linux UDP
+/// GSO: `WSASendMsg` + `UDP_SEND_MSG_SIZE`. **On by default** (the 1 Gbps+ send lever — Windows
+/// otherwise does one `send` syscall per packet, which caps throughput at high packet rates). Kill
+/// switch `PUNKTFUNK_GSO=0`; auto-fallback latches it off the first time a send reports it
+/// unsupported (old OS / NIC / path). We detect support from the send error rather than a
+/// `setsockopt` probe — the probe sets a socket-wide default segment size that would fragment plain
+/// `send`s of larger-than-segment packets.
+#[cfg(target_os = "windows")]
+mod uso {
+    use std::sync::atomic::{AtomicU8, Ordering};
+    static STATE: AtomicU8 = AtomicU8::new(0); // 0 = uninit, 1 = on, 2 = off
+
+    pub fn active() -> bool {
+        match STATE.load(Ordering::Relaxed) {
+            1 => true,
+            2 => false,
+            _ => {
+                let off = std::env::var_os("PUNKTFUNK_GSO")
+                    .map(|v| v == "0")
+                    .unwrap_or(false);
+                STATE.store(if off { 2 } else { 1 }, Ordering::Relaxed);
+                tracing::info!(
+                    enabled = !off,
+                    "Windows UDP Send Offload (USO): {} (the 1 Gbps+ send lever; PUNKTFUNK_GSO=0 disables)",
+                    if off { "off" } else { "on" }
+                );
+                !off
+            }
+        }
+    }
+    /// Latch USO off for the process after a send that means it isn't usable on this OS/NIC/path.
+    pub fn disable() {
+        if STATE.swap(2, Ordering::Relaxed) != 2 {
+            tracing::warn!("Windows USO unsupported on this path — falling back to per-packet sends");
+        }
+    }
+}
+
+/// True if a `WSASendMsg` USO error means USO isn't usable here (vs a transient full-buffer
+/// `WouldBlock`, handled by [`is_transient_io`]) — latch it off and fall back to per-packet sends.
+/// 10022 WSAEINVAL, 10042 WSAENOPROTOOPT, 10045 WSAEOPNOTSUPP, 10040 WSAEMSGSIZE.
+#[cfg(target_os = "windows")]
+fn uso_unsupported(e: &std::io::Error) -> bool {
+    matches!(
+        e.raw_os_error(),
+        Some(10022) | Some(10042) | Some(10045) | Some(10040)
+    )
+}
+
+/// One `WSASendMsg` carrying a `UDP_SEND_MSG_SIZE` control message: Winsock splits `buf` (a
+/// back-to-back concatenation of equal-size datagrams, only the final one allowed shorter) into
+/// `seg_size`-byte UDP datagrams to the connected peer in ONE syscall — the analogue of
+/// [`send_one_gso`]. The `WSA_CMSG_*` helpers are C macros not exported by the `windows` crate, so
+/// the cmsg layout math is reimplemented here (ported from quinn-udp's Windows backend).
+#[cfg(target_os = "windows")]
+fn send_one_uso(socket: &std::net::UdpSocket, buf: &[u8], seg_size: u16) -> std::io::Result<()> {
+    use std::os::windows::io::AsRawSocket;
+    use windows_sys::Win32::Networking::WinSock::{
+        WSASendMsg, CMSGHDR, IPPROTO_UDP, UDP_SEND_MSG_SIZE, WSABUF, WSAMSG,
+    };
+    let align_usize = std::mem::align_of::<usize>();
+    let align_hdr = std::mem::align_of::<CMSGHDR>();
+    let cmsgdata_align = |n: usize| (n + align_usize - 1) & !(align_usize - 1);
+    let cmsghdr_align = |n: usize| (n + align_hdr - 1) & !(align_hdr - 1);
+    let hdr = std::mem::size_of::<CMSGHDR>();
+
+    // 8-byte-aligned control buffer; 32 B holds one u32 cmsg (WSA_CMSG_SPACE(4) = 24 on x64).
+    #[repr(align(8))]
+    struct Aligned([u8; 32]);
+    let mut ctrl = Aligned([0u8; 32]);
+
+    let mut data = WSABUF {
+        len: buf.len() as u32,
+        buf: buf.as_ptr() as *mut u8, // WSASendMsg only reads it
+    };
+    let mut msg = WSAMSG {
+        name: std::ptr::null_mut(),
+        namelen: 0,
+        lpBuffers: &mut data,
+        dwBufferCount: 1,
+        Control: WSABUF {
+            len: 0,
+            buf: ctrl.0.as_mut_ptr(),
+        },
+        dwFlags: 0,
+    };
+    let cmsg_len = cmsgdata_align(hdr) + std::mem::size_of::<u32>(); // WSA_CMSG_LEN(4)
+    let space = cmsgdata_align(hdr + cmsghdr_align(std::mem::size_of::<u32>())); // WSA_CMSG_SPACE(4)
+    unsafe {
+        let cmsg = ctrl.0.as_mut_ptr() as *mut CMSGHDR;
+        (*cmsg).cmsg_len = cmsg_len;
+        (*cmsg).cmsg_level = IPPROTO_UDP;
+        (*cmsg).cmsg_type = UDP_SEND_MSG_SIZE;
+        let data_ptr = (cmsg as usize + cmsgdata_align(hdr)) as *mut u32;
+        std::ptr::write_unaligned(data_ptr, seg_size as u32);
+        msg.Control.len = space as u32;
+        let mut sent = 0u32;
+        let rc = WSASendMsg(
+            socket.as_raw_socket() as usize,
+            &msg,
+            0,
+            &mut sent,
+            std::ptr::null_mut(),
+            None,
+        );
+        if rc != 0 {
+            return Err(std::io::Error::last_os_error());
+        }
+    }
+    Ok(())
+}
+
 /// Apple (macOS/iOS) batched-receive enable state. Darwin has no `recvmmsg(2)`, so without this our
 /// macOS client does one `recv` syscall per packet — at a few hundred Mbps that's ~40-90k syscalls/s
 /// on one core, and when the recv loop can't drain fast enough the kernel socket buffer backs up and
@@ -487,6 +599,51 @@ impl Transport for UdpTransport {
         Ok(sent)
     }
 
+    /// UDP USO send (see [`Transport::send_gso`]) — Windows. Coalesces the frame's equal-size packets
+    /// and hands Winsock ≤512-segment super-buffers via `WSASendMsg(UDP_SEND_MSG_SIZE)` — one syscall
+    /// per chunk instead of one `send` per packet, the 1 Gbps+ lever (Windows analogue of Linux GSO).
+    /// On by default (kill: `PUNKTFUNK_GSO=0`); falls back to the scalar `send_batch` when off, when
+    /// packets aren't uniform-size, or on a USO-unsupported error (which latches it off for the
+    /// process). Same lossy short-count contract.
+    #[cfg(target_os = "windows")]
+    fn send_gso(&self, packets: &[&[u8]]) -> std::io::Result<usize> {
+        if packets.is_empty() {
+            return Ok(0);
+        }
+        if !uso::active() {
+            return self.send_batch(packets);
+        }
+        // USO needs every segment but the last to be exactly `seg` bytes (same as Linux GSO).
+        let seg = packets[0].len();
+        let last = packets.len() - 1;
+        if seg == 0 || packets[..last].iter().any(|p| p.len() != seg) || packets[last].len() > seg {
+            return self.send_batch(packets);
+        }
+        // Win11 x64 accepts up to ~512 segments per WSASendMsg.
+        let max_seg = 512usize;
+        let mut scratch: Vec<u8> = Vec::with_capacity(seg * packets.len().min(max_seg));
+        let mut sent = 0usize;
+        for chunk in packets.chunks(max_seg) {
+            scratch.clear();
+            for p in chunk {
+                scratch.extend_from_slice(p);
+            }
+            match send_one_uso(&self.socket, &scratch, seg as u16) {
+                Ok(()) => sent += chunk.len(),
+                // Send buffer momentarily full / connected-socket ICMP blip — drop the rest, never
+                // block, never tear down (see is_transient_io).
+                Err(e) if is_transient_io(&e) => break,
+                // USO unsupported on this OS/NIC/path — latch off and finish via scalar send_batch.
+                Err(e) if uso_unsupported(&e) => {
+                    uso::disable();
+                    return Ok(sent + self.send_batch(&packets[sent..])?);
+                }
+                Err(e) => return Err(e),
+            }
+        }
+        Ok(sent)
+    }
+
     fn recv(&self) -> std::io::Result<Option<Vec<u8>>> {
         let mut buf = vec![0u8; RECV_BUF];
         match self.socket.recv(&mut buf) {