windows: Watch does not fire on image (Ctrl+C) or file copy

[changkun]

Summary

On Windows, Watch does not fire for two common clipboard actions users expect to observe:

1. Copying an image via Ctrl+C in some apps (only screenshot flows like Shift+Win+S are reliably detected).
2. Copying files in Explorer — file copies place a file-drop/CF_HDROP payload on the clipboard, which the library has no format for, so the change is invisible to Watch.

Impact

Users building clipboard monitors miss image and file-copy events on Windows, which is surprising given Watch works for text/PNG.

History

Reported across #50 and #37. Both were closed as out-of-scope/by-design at the time (only FmtText/FmtImage were supported). Re-filing as a tracked enhancement because the recent custom-formats work (#17) now provides the registry mechanism that could expose these payloads (e.g. a registered CF_HDROP / file-list format).

Proposed direction

• Evaluate exposing CF_HDROP (file list) as a registered custom format on Windows so Watch/Read can observe file copies.
• Confirm whether image copies via Ctrl+C raise a clipboard sequence-number change that Watch should already catch, and fix if not.

Enhancement, platform-specific (Windows).