feat(snap,pick): eye-relative CPU projection — drop full view_proj at UTM

Route the remaining absolute-coordinate CPU projection through the
relative-to-eye path so it no longer multiplies by a view_proj matrix
carrying a ~1e7 translation (which cancels catastrophically in f32 after
world_offset removal):
- camera pick_on_plane / pick_on_target_plane delegate to
  unproject_on_plane_f64 (rotation-only inverse + f64 eye)
- snapper (world_to_screen), hit_test (click_hit / mesh / hatch), the
  scene mesh/solid hit methods, grid + UCS axes, and polar_constrain_near
  all take view_rot + eye and project relative-to-eye
- compute_grid_step measures the per-unit scale via the rotation-only
  matrix (no eye term needed)

view_proj remains only for conservative CPU frustum-cull / scissor and the
screen-space viewcube / UCS-icon directions. NOTE: SnapResult.world and
the command-point chain are still Vec3 (f32) — a ~0.5 m quantization at
extreme UTM zoom remains; the command layer goes f64 next.

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

src/app/helpers.rs

@@ -228,14 +228,15 @@ pub(super) fn polar_constrain_near(
     pt: glam::Vec3,
     base: glam::Vec3,
     step_deg: f32,
-    view_proj: glam::Mat4,
+    view_rot: glam::Mat4,
+    eye: glam::DVec3,
     bounds: iced::Rectangle,
     tol_px: f32,
     xf: &UcsXform,
 ) -> glam::Vec3 {
     let snapped = polar_constrain(pt, base, step_deg, xf);
     let to_screen = |w: glam::Vec3| {
-        let ndc = view_proj.project_point3(w);
+        let ndc = view_rot.project_point3((w.as_dvec3() - eye).as_vec3());
         (
             (ndc.x + 1.0) * 0.5 * bounds.width,
             (1.0 - ndc.y) * 0.5 * bounds.height,