Add win32 platform branch to clipboard.py so Ctrl+V image paste
works on native Windows (PowerShell / Windows Terminal), not just
WSL2.
Uses the same .NET System.Windows.Forms.Clipboard approach as the
WSL path but calls PowerShell directly instead of powershell.exe
(the WSL cross-call path). Tries 'powershell' first (Windows
PowerShell 5.1, always available), then 'pwsh' (PowerShell 7+).
PowerShell executable is discovered once and cached for the process
lifetime.
Includes 14 new tests covering:
- Platform dispatch (save_clipboard_image + has_clipboard_image)
- Image detection via PowerShell .NET check
- Base64 PNG extraction and decode
- Edge cases: no PowerShell, empty output, invalid base64, timeout
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