feat: Windows backend (WinCairo WebKit) + docs

[ipfizz]

A from-scratch Windows backend on pure bun:ffi - and WinCairo WebKit, never WebView2/Chromium.

What's built (~6k LOC, packages/bunmaska/src/main/platform/windows/)

• Win32 foundation: UTF-16 marshalling, import-safe DLL loaders, a native window + cooperative message pump (non-blocking, cooperates with Bun like the macOS/Linux pumps).
• WinCairo WebKit view in an HWND: the WebKit2 C-API loader (WebKit2.dll, not COM/WebView2), renderer↔main IPC bridge, navigation lifecycle, zoom/user-agent/auto-resize. Loaded from the engine store via SetDllDirectory (the Windows analogue of $ORIGIN/@rpath).
• ~10 modules: clipboard (text+HTML), safeStorage (DPAPI), tray (Shell_NotifyIcon), screen (EnumDisplayMonitors + per-monitor DPI), shell (ShellExecuteW), globalShortcut (RegisterHotKey), powerSaveBlocker (SetThreadExecutionState), powerMonitor (suspend/resume/lock), nativeTheme (registry dark mode).
• CLI: bunmaska build --target windows → a compiled .exe + engine.id + a .zip.
• Platform: windows is now a supported platform; x64 + ARM64, never x86. A windows-latest CI leg runs validate:windows.

Honest gaps (throw a clear error, don't silently no-op): printToPDF/capturePage, DevTools, clipboard images, tray context menu. End-to-end shipping still needs a hosted WinCairo engine (same blocker as the Linux pinned tier).

Docs updated to match: roadmap (Windows → "in the build"), platforms page, homepage, README. Plus a small theme-color meta.

🤖 Generated with Claude Code