DWF/DGN underlays: draw content as vector wires

A DWF sheet or DGN model is drawn as world-space wires (one line wire per
colour and width, one fill wire per colour) instead of a raster rescaled
to the view, so it stays sharp while zooming instead of showing pixels
until the raster is made again. Colours go through the colour table
mapping, contrast, fade and monochrome; clip boundaries clip the wires.
The wires are drawn and plotted but neither picked nor snapped; the
underlay's own snap geometry still serves object snaps.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/snap.rs

@@ -1349,7 +1349,7 @@ impl Snapper {
         let unindexed = wires.segments().is_none();
         if unindexed {
             for w in wires.iter() {
-                if wire_in_range(w) {
+                if !w.is_display_only() && wire_in_range(w) {
                     in_range_pts += w.points.len();
                     in_range_wires.push(w);
                 }