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>

tests/block_hatch_export.rs

@@ -77,7 +77,7 @@ fn block_internal_hatch_reaches_export() {
     // minimal shape of "coloured fill nested in a block".
     let h = scene.add_entity(EntityType::Hatch(square_hatch(5)));
     scene
-        .create_block_from_entities(&[h], "LOGO", glam::Vec3::ZERO)
+        .create_block_from_entities(&[h], "LOGO", glam::DVec3::ZERO)
         .expect("wrap hatch into a block + insert");
     scene.populate_hatches_from_document();