Add vault locking and a cross-platform build script

Commands that change the vault hold an exclusive lock on DIR/.lock and
readers hold a shared one, taken after the passphrase prompt. A blocked
process says it is waiting. Unix uses fcntl locks and Windows uses
LockFileEx, both released by the OS if the process dies. Plan 9 and
WebAssembly fall back to an exclusive lock file.

build.sh cross-compiles static binaries for every Go target that builds
without cgo (43 today, armv6 and armv7 for 32-bit ARM Linux) and writes
SHA256SUMS.

lock_windows.go

@@ -0,0 +1,36 @@
+// Copyright 2026 The vault Authors.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+//go:build windows
+
+package main
+
+import (
+	"errors"
+	"os"
+
+	"golang.org/x/sys/windows"
+)
+
+// acquireLock uses LockFileEx, which Windows releases if the process dies.
+func acquireLock(path string, exclusive bool, waiting func()) (func(), error) {
+	f, err := os.OpenFile(path, os.O_RDWR|os.O_CREATE, 0o600)
+	if err != nil {
+		return nil, err
+	}
+	h := windows.Handle(f.Fd())
+	var flags uint32
+	if exclusive {
+		flags = windows.LOCKFILE_EXCLUSIVE_LOCK
+	}
+	err = windows.LockFileEx(h, flags|windows.LOCKFILE_FAIL_IMMEDIATELY, 0, 1, 0, new(windows.Overlapped))
+	if errors.Is(err, windows.ERROR_LOCK_VIOLATION) {
+		waiting()
+		err = windows.LockFileEx(h, flags, 0, 1, 0, new(windows.Overlapped))
+	}
+	if err != nil {
+		f.Close()
+		return nil, err
+	}
+	return func() { f.Close() }, nil
+}