Windows: build and run natively

[endel]

Summary

quic-zig now builds, tests and runs natively on Windows, and CI gains a windows-latest job.

• Syscall seam. src/sys.zig had a compile error on every Windows branch. Windows now lives in src/sys/windows.zig: Winsock for sockets (Zig 0.16 dropped the ws2_32 bindings, so the few we call are declared there), std.Io for files, clocks, randomness and sleep. The 20 per-function OS switches became one supported-OS check.
• Helpers instead of repeated POSIX setup. sys.udpSocket, setsockopt, setReuseAddr, stdout and argsIterator replace socket setup, IPV6_V6ONLY magic numbers and argument parsing that were copied across src/ and 26 apps.
• Event loop. It waits for readiness (xev.File.poll), which libxev's IOCP backend lacked. build.zig.zon pins endel/libxev@14899db, which adds poll through the AFD driver — proposed upstream in iocp: add poll, through the AFD driver mitchellh/libxev#240. Once that merges, the pin can go back to upstream.

Windows behaviour worth knowing

Documented in SPEC/STATUS.md#platforms:

• A UDP socket no longer fails its next receive after an ICMP port-unreachable (SIO_UDP_CONNRESET), so one departed client cannot break a server's receive loop.
• SO_REUSEADDR is not set: on Windows it lets another socket take a port that is in use.
• ECN is neither read nor set; there is no send batching or UDP offload.
• Sleeps and timed waits round to the ~15.6 ms system tick, which makes the manual-loop bench app slow on Windows.

Testing

• Windows (Zig 0.16.0): zig build test 757/757, zig build fuzz 612/612, every app builds; server + bench completed 1000/1000 requests in 1-RTT and 0-RTT.
• Linux (WSL, Zig 0.16.0): zig build test 757/757, every app builds.
• macOS: not run locally — covered by CI.

WebTransport echo latency, wt-echo-server (ReleaseFast) vs quic-go v0.59.0 wt_server, same Go wt_latency client, loopback, medians:

	quic-zig datagram	quic-go datagram	quic-zig bidi	quic-go bidi
Windows (5×1000)	117–214 µs	147–226 µs	132–230 µs	179–312 µs
Linux (3×1000)	137–139 µs	195–202 µs	152–155 µs	239–248 µs

On Windows the client timed with QueryPerformanceCounter; Go's own clock there is too coarse for sub-millisecond round trips.

🤖 Generated with Claude Code

https://claude.ai/code/session_01YXLKbmpkRS5DedisNs9VNp