fix(precision): keep BLOCK base and TEXT/MTEXT insertion points in f64 (#311)

Three commit helpers narrowed a typed/picked point back to f32 before it
reached the persisted entity, so the coordinate was quantized to the f32
grid (worse at large/UTM extents):

- Scene::create_block_from_entities took the base point as glam::Vec3, so
  the block's local-space translation and the replacement INSERT's
  insert point were f32. Param is now DVec3 (matching the sibling
  define_block_from_owned_entities); the command_driver handler no longer
  downcasts with as_vec3().
- The MTEXT and TEXT in-place editors stored `pos` as Vec3 and built the
  committed insertion_point from it, quantizing a new label's position
  and re-quantizing an existing one on edit. Both `pos` fields are now
  DVec3.

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

src/app/command_driver.rs

@@ -378,7 +378,7 @@ impl OpenCADStudio {
                 self.push_undo_snapshot(i, "BLOCK");
                 match self.tabs[i]
                     .scene
-                    .create_block_from_entities(&handles, &name, base.as_vec3())
+                    .create_block_from_entities(&handles, &name, base)
                 {
                     Ok(insert_handle) => {
                         self.tabs[i].dirty = true;
@@ -1858,7 +1858,7 @@ impl OpenCADStudio {
             } => {
                 self.tabs[i].active_cmd = None;
                 self.tabs[i].snap_result = None;
-                self.open_mtext_editor(pos.as_vec3(), handle, &initial, height);
+                self.open_mtext_editor(pos, handle, &initial, height);
             }
             CmdResult::OpenTextEditor {
                 pos,
@@ -1869,7 +1869,7 @@ impl OpenCADStudio {
                 self.tabs[i].active_cmd = None;
                 self.tabs[i].snap_result = None;
                 self.open_text_inline(
-                    pos.as_vec3(),
+                    pos,
                     handle,
                     &initial,
                     height,