* CPItemProvider.j: Introduce CPItemProvider, modelled on NSItemProvider.
- Register data representations keyed by MIME type, with wildcard matching.
- Load representations asynchronously, completing inside a run loop pass.
- Read browser File objects with FileReader.
- Document remaining NSItemProvider compatibility gaps via TODO comments.
* Foundation.j: Import CPItemProvider.j.
* CPPlatformFilePanel.j: Introduce the DOM platform class that runs the browser's file dialog through a hidden file input.
* CPOpenPanel.j: Replace the NativeHost implementation with a browser implementation.
- Implement -beginWithCompletionHandler: and -beginSheetModalForWindow:completionHandler:.
- Add allowed file types, message, prompt, and an accessory view behind an Options button.
- Validate chosen files through the window delegate's panel:validateURL:error:.
- Add -itemProviders, and -URLs holding file names.
- Raise an exception from -runModal.
- Document remaining NSOpenPanel compatibility gaps via TODO comments.
* CPTextView.j: Insert images through CPOpenPanel and CPItemProvider, for both -insertImage: and paste.
* CPItemProviderTest.j, CPOpenPanelTest.j: Add unit tests covering registration, loading, type matching, panel results, and cancelled runs.
