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/scene/pick/hit_test.rs

@@ -759,9 +759,10 @@ mod aabb_reject_tests {
         let near = wire("5", vec![[-0.02, 0.0, 0.0], [0.02, 0.0, 0.0]], [-0.02, 0.0, 0.02, 0.0]);
         let far = wire("9", vec![[0.9, 0.9, 0.0], [0.95, 0.9, 0.0]], [0.9, 0.9, 0.95, 0.9]);
 
-        assert_eq!(click_hit(cursor, std::slice::from_ref(&near), vp, bounds), Some("5"));
-        assert_eq!(click_hit(cursor, std::slice::from_ref(&far), vp, bounds), None);
+        let eye = glam::DVec3::ZERO;
+        assert_eq!(click_hit(cursor, std::slice::from_ref(&near), vp, eye, bounds), Some("5"));
+        assert_eq!(click_hit(cursor, std::slice::from_ref(&far), vp, eye, bounds), None);
         // The far wire must be rejected without hiding the near one.
-        assert_eq!(click_hit(cursor, &[far, near], vp, bounds), Some("5"));
+        assert_eq!(click_hit(cursor, &[far, near], vp, eye, bounds), Some("5"));
     }
 }