refactor(snap): widen snap_pts to f64 (world_offset removal P2)

WireModel/LocalWire/TruckEntity snap buffers and the producers/consumers
now carry snap candidates as DVec3 (offset_snap_pts, dimension/multileader
producers, block-cache transform, pick/xclip, snap module). Still
offset-relative, so behaviour is unchanged — this just lets the snap
transport survive the upcoming switch to absolute coordinates without an
extra f32 quantization. The snap module still resolves in f32 (as_vec3 at
entry), matching today's precision.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/scene/pick/xclip.rs

@@ -104,7 +104,7 @@ pub fn clip_wires(wires: &mut Vec<WireModel>, poly: &[[f32; 2]]) {
         }
         w.key_vertices
             .retain(|v| point_in_poly(v[0], v[1], poly));
-        w.snap_pts.retain(|(p, _)| point_in_poly(p.x, p.y, poly));
+        w.snap_pts.retain(|(p, _)| point_in_poly(p.x as f32, p.y as f32, poly));
         w.aabb = recompute_aabb(&w.points, &w.fill_tris);
     }
     wires.retain(|w| !w.points.is_empty() || !w.fill_tris.is_empty());