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