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/scene/entity.rs

@@ -250,7 +250,7 @@ impl Scene {
         &mut self,
         handles: &[Handle],
         name: &str,
-        base: glam::Vec3,
+        base: glam::DVec3,
     ) -> Result<Handle, String> {
         let name = name.trim();
         if name.is_empty() {
@@ -299,7 +299,7 @@ impl Scene {
             .add_entity(EntityType::BlockEnd(block_end))
             .map_err(|e| e.to_string())?;
 
-        let local = EntityTransform::Translate((-base).as_dvec3());
+        let local = EntityTransform::Translate(-base);
         for (old_handle, mut entity) in source_entities {
             view::dispatch::apply_transform(&mut entity, &local);
             entity = crate::modules::draw::modify::explode::normalize_entity_for_block(entity);
@@ -313,7 +313,7 @@ impl Scene {
 
         let insert = DxfInsert::new(
             name,
-            acadrust::types::Vector3::new(base.x as f64, base.y as f64, base.z as f64),
+            acadrust::types::Vector3::new(base.x, base.y, base.z),
         );
         Ok(self.add_entity(EntityType::Insert(insert)))
     }