fix(paste): UTM-precise paste/block base via f64 centroid

entities_centroid summed the f32 points (double-single HIGH half) alone, so
at UTM scale (~5.7e6) it both quantized each term ~0.5 m and lost low bits
across the running total — drifting the paste base and PASTEBLOCK's block
base metres off, which placed the wrapped block at the wrong spot/scale.

It now reconstructs each vertex's absolute f64 from points + points_low and
accumulates in f64. clipboard_centroid is DVec3; the PASTECLIP commit delta
is fully f64; define_block_from_owned_entities takes an f64 base so the
block-local fold (Translate(-base)) stays precise. The interactive ghost
anchors (PasteCommand, InsertBlockCommand) still take the f32 cast — display
only; the committed geometry is f64.

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

src/scene/mod.rs

@@ -4204,7 +4204,7 @@ impl Scene {
         &mut self,
         entities: Vec<EntityType>,
         name: &str,
-        base: glam::Vec3,
+        base: glam::DVec3,
     ) -> Result<(), String> {
         let name = name.trim();
         if name.is_empty() {
@@ -4248,7 +4248,7 @@ 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 mut entity in entities {
             view::dispatch::apply_transform(&mut entity, &local);
             entity = crate::modules::draw::modify::explode::normalize_entity_for_block(entity);