feat(connect): Windows capture/input backend for Crabfleet Connect

[steipete]

Summary

Completes the Crabfleet Connect cross-platform pair — adds a Windows capture/input backend to the shared Go host core (Linux landed in #101). Foundation, same honesty bar as Linux: compiles and cross-builds; real-hardware capture/input is not validated (no Windows box available).

• internal/connect/windows.go (behind //go:build windows): pure-Go via syscall.NewLazyDLL against user32/gdi32 — no new dependency. GDI BitBlt primary-display capture (synchronized read, top-down DIB → RGBA, full-frame dirty rect). SendInput for absolute pointer, buttons, vertical/horizontal wheel, and keyboard.
• Keymap: named keys via VK codes, printable text via Unicode injection, Ctrl/Alt/Win shortcuts through the active Windows layout; AltGr, legacy X11 keysyms, and surrogate pairs handled. The keysym→VK table is in a non-tagged file so it's unit-tested on any OS.
• platform_windows.go selects the backend on Windows; synthetic fallback elsewhere. cmd/crabfleet-connect and READMEs updated.
• No wire-protocol change; the existing Capturer/InputSink contracts are unchanged, so the Windows host speaks the same RFB the macOS/web clients already decode.

Deferred (documented)

DXGI Desktop Duplication, multi-monitor, per-monitor DPI, service packaging, and real-hardware validation.

Proof

• go build/go vet/go test ./...: 170 pass (incl. OS-independent keymap tests)
• Cross-compile: windows/amd64 and windows/arm64 → valid PE32+
• pnpm test 969/969 (no TS touched); autoreview clean