feat(#153): compile-level Windows support + CI guard

[justrach]

Closes #153.

Summary

Adds compile-level Windows support: zig build -Dtarget=x86_64-windows-gnu -Doptimize=ReleaseSafe produces all four user-facing PE32+ binaries (kuri.exe, kuri-agent.exe, kuri-browse.exe, kuri-fetch.exe) plus merjs-e2e.exe. A new windows-cross-compile CI job pins this baseline so a future refactor can't quietly re-introduce an ungated POSIX call (fork, std.c.open, raw setsockopt/read on a socket) without breaking CI.

macOS and Linux builds are unchanged; this is additive.

Commits

1. docs(install): mark Windows unsupported and link tracking issue #153 — Original "punt" commit: a Platform-support table in readme.md, plus MINGW*/MSYS*/CYGWIN* detection in install.sh and a win32 branch in npm/bin/install.js so users get a tailored error pointing at this issue instead of a generic Unsupported OS. (The npm self-test still passes — the existing /unsupported platform: win32/ assertion still matches the new message.)
2. refactor(compat): cross-platform realtime clock + isatty (towards #153) — Routes the three time wrappers in src/compat.zig through a single realtimeNanos() that picks GetSystemTimeAsFileTime on Windows and clock_gettime on POSIX. Adds compat.isTtyStderr() (GetConsoleMode on Windows, isatty(2) on POSIX). Switches the three call sites to it.
3. feat(compat): compile-level Windows support — The big one. Adds enough cross-platform / stub code to make every Zig-side POSIX assumption Windows-compatible.
4. ci(windows): cross-compile guard + docs update — The CI job + a more accurate readme entry ("Experimental — cross-compile only").

What works on Windows

• Realtime clock, monotonic sleep, TTY detection (compat.zig).
• Mutex / RwLock via Win32 SRWLOCK instead of pthread.
• Stdout/stderr writes via WriteFile(GetStdHandle(...)).
• --version / --help paths through main for each binary.
• Pure in-memory operations.

What's stubbed with error.UnsupportedOnWindows

These return an error at runtime, they do not silently no-op:

• compat.cwd* file ops, compat.runCommand, compat.tcp* / TcpStream / TcpServer (raw BSD-socket code).
• chrome/launcher.zig Chrome spawn + reaper (fork + SIG.KILL).
• cdp/websocket.zig connect + raw socket read; cdp/client.zig drainWsEvents.
• server/router.zig discoverTabs (raw setsockopt/read on the CDP socket) — gets an explicit DiscoverTabsError so the handler's switch stays exhaustive on both platforms.
• storage/auth_profiles.zig deleteProfile / listProfiles (POSIX opendir/readdir).
• server/api_token.zig writeMode0600 (POSIX open + fchmod).
• agent_main.zig cmdOpen (fork+exec Chrome) and fetchChromeTabs (raw socket).
• browse_main.zig repl (POSIX stdin fd) and readLine (std.posix.read).
• lifecycle.zig install (POSIX sigaction).

Out of scope for this PR (follow-up Windows work)

• Real Chrome launching on Windows (CreateProcess + JobObject for cleanup).
• Daemonization (CreateProcess with DETACHED_PROCESS).
• Real Win32 socket implementations for the CDP path so Chrome automation works.
• Signal-equivalent shutdown handling (SetConsoleCtrlHandler).
• Windows release pipeline + manifest entry + npm installer asset.
• A Windows runtime smoke test in CI (would need a windows-latest job, not just cross-compile).

Test plan

• zig build clean on macOS (Apple Silicon, Zig 0.16.0).
• zig build -Dtarget=x86_64-windows-gnu clean on macOS — produces 5 PE32+ x86_64 binaries.
• node -e "require('./npm/bin/install.js').runSelfTest()" passes.
• sh -n install.sh clean.
• CI: existing Linux build + tests pass.
• CI: new windows-cross-compile job passes.
• Runtime: kuri.exe --version on a real Windows box (or under Wine) — out of scope for me to verify here.

🤖 Generated with Claude Code