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>

Cargo.lock

@@ -2609,6 +2609,7 @@ dependencies = [
  "thiserror 2.0.18",
  "tokio",
  "tracing",
+ "windows-sys 0.59.0",
  "zerocopy",
  "zeroize",
 ]
@@ -4375,6 +4376,15 @@ dependencies = [
  "windows-targets 0.52.6",
 ]
 
+[[package]]
+name = "windows-sys"
+version = "0.59.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1e38bc4d79ed67fd075bcc251a1c39b32a1776bbe92e5bef1f0bf1f8c531853b"
+dependencies = [
+ "windows-targets 0.52.6",
+]
+
 [[package]]
 name = "windows-sys"
 version = "0.60.2"