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/scene/model/wire_model.rs

@@ -506,6 +506,9 @@ pub struct WireModel {
     pub depth_override: Option<f32>,
     /// Whether this wire is drawn in the normal viewport pass.
     pub display_visible: bool,
+    /// Geometry that only object snaps use (the content of an underlay):
+    /// never drawn, plotted or picked.
+    pub snap_only: bool,
     /// Whether this wire is included in plotted output.
     pub plot_visible: bool,
     /// `true` when [`fill_tris`] is a real 3-D surface (PolyfaceMesh /
@@ -580,6 +583,7 @@ impl WireModel {
             world_width: 0.0,
             depth_override: None,
             display_visible: true,
+            snap_only: false,
             plot_visible: true,
             fill_is_3d: false,
             fill_is_2d_solid: false,
@@ -1009,6 +1013,7 @@ impl Default for WireModel {
             fill_tris_low: Vec::new(),
             depth_override: None,
             display_visible: true,
+            snap_only: false,
             plot_visible: true,
             fill_is_3d: false,
             fill_is_2d_solid: false,