Implement proper file locking for Windows

[dapi]

Problem

Currently, Windows builds have no file locking:

// lock_windows.go
func (l *FileLock) Lock() error {
    once.Do(func() {
        fmt.Fprintln(os.Stderr, "warning: file locking not available on Windows...")
    })
    return nil
}

This means concurrent port-selector invocations on Windows can corrupt the allocations file.

Impact

• Data corruption risk with parallel agents on Windows
• Warning message on every run (via sync.Once per process)
• Windows is a second-class platform

Proposed Solution

Implement Windows file locking using LockFileEx:

// lock_windows.go
import "golang.org/x/sys/windows"

func (l *FileLock) Lock() error {
    var overlapped windows.Overlapped
    err := windows.LockFileEx(
        windows.Handle(l.file.Fd()),
        windows.LOCKFILE_EXCLUSIVE_LOCK,
        0,
        1, 0,
        &overlapped,
    )
    return err
}

func (l *FileLock) Unlock() error {
    var overlapped windows.Overlapped
    return windows.UnlockFileEx(
        windows.Handle(l.file.Fd()),
        0,
        1, 0,
        &overlapped,
    )
}

Trade-offs

Pros:

• Full Windows support
• No more warning messages
• Safe concurrent access

Cons:

• New dependency: golang.org/x/sys/windows
• Need Windows environment for testing
• Increases complexity

Alternative

If Windows is not a priority platform, document this limitation clearly in README and keep current behavior.

Acceptance Criteria

• Decide: implement or document limitation
• If implement: add golang.org/x/sys/windows dependency
• If implement: write Windows-specific locking code
• If implement: test on Windows (CI or manual)
• Update README with Windows status