OxyPixel 0.4.0

Added

• Copy puts the selection on the system clipboard as a straight-alpha image and keeps the in-memory patch. Paste paints a system image when one is present, or that patch when the clipboard has none. Ctrl+C and Ctrl+V still run when the text clipboard is empty. On Windows the image is a PNG and a 32-bit DIB. On Linux it is image/png while OxyPixel is open.
• Linux releases include oxypixel-0.4.0-linux-x86_64.deb beside the tar.gz. The package installs /usr/bin/oxypixel, a desktop entry, a 32×32 icon, and the .oxypix file type. Check for updates downloads that package when the program is /usr/bin/oxypixel, and still downloads the tar.gz for an unpacked copy. Install the downloaded package with the package manager.
• The palette can be imported and exported as GIMP .gpl, a hex list (.hex or .txt, one RRGGBB per line, or RRGGBBAA when the color has alpha), and JASC .pal. Import replaces the palette and is one undo step. export-palette and import-palette do the same from the command line.
• cargo test compares the built-in example's composited frames with a stored image hash. It does not open a window.

Changed

• The README is written for people downloading and running the editor, with screenshots of the window.
• Tests lock the update filenames to the version of this copy, and check that recent files use the platform data directory.
• make check-windows runs clippy for the Windows build, and the Ubuntu CI job runs that check.
• A pull request runs CI once. A newer push on the same branch cancels the run it replaces. Documentation-only changes skip the test jobs. Tag pushes stay on the release workflow.