fix: make X11/Wayland backends Linux-only so GOOS=windows builds clean

[tannevaled]

Problem

The module did not compile for GOOS=windows (or any non-unix GOOS). internal/x11 and internal/wayland used unix-only syscalls that were not build-guarded, so go build ./... compiled them everywhere and failed:

# github.com/go-widgets/window/internal/wayland
internal/wayland/transport.go:31:23: undefined: syscall.CmsgSpace
internal/wayland/seat.go:298:18:     undefined: syscall.Mmap
internal/wayland/shm.go:89:24:       undefined: syscall.Mmap
# github.com/go-widgets/window/internal/x11
internal/x11/conn.go:232:17:         undefined: syscall.UnixRights
internal/x11/shmbuf.go:63:18:        undefined: syscall.Mmap
internal/x11/shmbuf.go:53:9:         cannot use fd (uintptr syscall.Handle) as int ...

The top-level window package already stubbed non-Linux (open_linux.go / open_other.go → ErrUnsupported), but the internal backends leaked unix syscalls into the windows build.

Fix

Extract only the OS-specific primitives into build-tagged files, keeping every protocol/codec type and all transport-agnostic logic cross-platform (so their unit tests still build/run on every GOOS — the design already described in open_other.go):

• internal/x11: syscalls_{linux,other}.go (anon file + mmap/munmap/close), conn_linux.go (unixRW SCM_RIGHTS fd passing)
• internal/wayland: syscalls_{linux,other}.go (anon file + shm mmap + keymap mmap), transport_linux.go (unixTransport SCM_RIGHTS), conn_linux.go (New over *net.UnixConn)

Shared-memory syscalls now sit behind package-var indirection: real on Linux, ErrUnsupported stubs off Linux. No live Linux behavior changes. Test harnesses needing a real socketpair/mmap are tagged //go:build linux (wayland helpers split into helpers_test.go + helpers_linux_test.go); socket-free codec/protocol tests stay cross-platform.

CI

• Coverage gate is now Linux-only (the backends are Linux-only).
• macOS lane proves the codec + ErrUnsupported stub build/vet/test off Linux.
• New cross-build matrix compiles windows/amd64, darwin/{arm64,amd64} and all six 64-bit Linux arches — the regression guard.

Verification (run, not asserted)

• go build ./... clean: windows/amd64, darwin/{arm64,amd64}, linux/{amd64,arm64,riscv64,loong64,ppc64le,s390x}
• CGO_ENABLED=0 go test ./... green on real linux/arm64 (Debian VM)
• 100.0% coverage on internal/x11 and internal/wayland
• go vet ./... clean on windows, darwin and linux (incl. -tags=integration)

🤖 Generated with Claude Code