v8.07.000: Save Flattened Image now includes zone mask fills + masked cells

- Explicit yellow filled painted region areas (from mask_images) are now composited in addition to base TIFF + paint_layer black boundaries + red cell mask.
- Uses alpha_composite for proper blending and visibility.
- Default filename is {name}_flattened.tif with correct initialdir.
- Both interactive Save Flattened and autosave updated.
- Overlays hardened (size/ndim guards, separate try blocks) so failures don't crash (base is always saved).
- Version bumped everywhere.
- New release notes, README, generator updated.

Author: blaingvt