fix(xclip): precise clip + carry clip across cross-drawing paste

Clipped blocks were broken at UTM and lost their clip when pasted into
another drawing.

- Clip geometry: world_clip_polygon_f64 keeps the boundary in absolute f64;
  clip_wires now clips in a frame relative to the boundary's first vertex
  (small coords → exact f32 Sutherland–Hodgman) and re-splits the result
  into the double-single high/low pair the relative-to-eye renderer needs,
  so the clipped block draws correctly and no longer poisons ZOOM Extents.
- ZOOM Extents (fit_all): the outlier reject compared absolute UTM
  coordinates against a centre-RELATIVE span (local_extent_max), rejecting
  the whole drawing → no-op. Made the reject distance-from-median-centre.
- Cross-drawing paste: clipboard now snapshots each copied entity's whole
  xdictionary object graph (ClipboardDeps.ext_objects) and recreates it on
  paste with fresh handles + remapped references, re-pointing the pasted
  entity's xdictionary at the new root. Wired into PASTECLIP and PASTEORIG.
  Fixed handle allocation (allocate_handle, not the non-advancing
  next_handle) so the dictionary chain doesn't collapse onto one handle.
- Updated hit_test / plugin_host tests for the eye-relative + DVec3 signatures.

Known gaps: PASTEBLOCK does not yet carry the clip (clip would need to live
inside the new wrapper block definition); hatch-clip path still f32.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/plugin_host.rs

@@ -356,7 +356,7 @@ mod tests {
             host.start_interactive(Box::new(PlacePoint { got_first: false }));
         }
         assert!(app.tabs[0].active_cmd.is_some());
-        for pt in [glam::Vec3::new(0.0, 0.0, 0.0), glam::Vec3::new(5.0, 5.0, 0.0)] {
+        for pt in [glam::DVec3::new(0.0, 0.0, 0.0), glam::DVec3::new(5.0, 5.0, 0.0)] {
             let r = app.tabs[0].active_cmd.as_mut().unwrap().on_point(pt);
             let _ = app.apply_cmd_result(r);
         }
@@ -410,7 +410,7 @@ mod tests {
             .active_cmd
             .as_mut()
             .unwrap()
-            .on_entity_pick(target, glam::Vec3::new(3.0, 4.0, 0.0));
+            .on_entity_pick(target, glam::DVec3::new(3.0, 4.0, 0.0));
         let _ = app.apply_cmd_result(r);
         // Original point + the mark the command committed.
         assert_eq!(app.tabs[0].scene.document.entities().count(), 2);