Frames: state-following ribbon list, DWF/DGN frames, image frame encoding and profile, frame-only picking

- The Frames list shows the drawing's frame state (hide / display and plot /
  display but not plot / frames vary), follows commands, undo and document
  switches, and lists the mixed state as a disabled item.
- DWFFRAME and DGNFRAME are frame variables of their own, kept in the header,
  and FRAME sets them with the others.
- Out-of-range frame values say "Requires an integer between 0 and 2." and
  ask again; a set value reports "Regenerating model.".
- IMAGEFRAME is stored in the image variables as bits (2 is written as 3) and
  a drawing without them uses the profile value, which setting IMAGEFRAME
  updates and the settings file keeps.
- Raster images, PDF underlays and wipeouts are picked on their frame only;
  underlay snap geometry is never picked.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/modules/insert/mod.rs

@@ -62,9 +62,12 @@ impl CadModule for InsertModule {
                             label: "",
                             icon: FRAMES_ICON,
                             items: vec![
-                                ("FRAMES0", "Frames Off", FRAMES_ICON),
-                                ("FRAMES1", "Frames On", FRAMES_ICON),
-                                ("FRAMES2", "Frames On, Not Plotted", FRAMES_ICON),
+                                ("FRAMES0", "Hide frames", FRAMES_ICON),
+                                ("FRAMES1", "Display and plot frames", FRAMES_ICON),
+                                ("FRAMES2", "Display but don't plot frames", FRAMES_ICON),
+                                // Shown (not selectable) while the frame
+                                // variables differ from each other.
+                                ("FRAMES3", "*Frames vary*", FRAMES_ICON),
                             ],
                             default: "FRAMES1",
                         },