feat(f64): command layer compiles on DVec3

Whole command/snap/grip/cursor chain now builds on f64 DVec3 end to end,
so picked points, snaps and grip drags stay precise at UTM-scale
coordinates instead of quantizing to ~0.5 m through f32.

- All ~80 command files migrated: draw (line/circle/arc/ellipse/shapes
  in full f64; others via point methods), modify, annotate, insert, view,
  inquiry, layers, groups, clipboard, layout.
- Entity apply_grip impls updated for GripApply::{Absolute,Translate}(DVec3).
- Construction sites (EntityTransform/CmdResult/DynAnchor) emit DVec3;
  wire-preview boundaries cast to f32 only at the GPU edge.
- Modify/annotate commands keep internal Vec3 geometry where precision is
  non-critical, casting at the DVec3 boundary; key draw commands store f64.

Builds clean (no warnings). Visual verification of crosshair/snap/grip at
extreme UTM zoom pending.

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

src/scene/mod.rs

@@ -4175,7 +4175,7 @@ impl Scene {
             .add_entity(EntityType::BlockEnd(block_end))
             .map_err(|e| e.to_string())?;
 
-        let local = EntityTransform::Translate(-base);
+        let local = EntityTransform::Translate((-base).as_dvec3());
         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);
@@ -4248,7 +4248,7 @@ impl Scene {
             .add_entity(EntityType::BlockEnd(block_end))
             .map_err(|e| e.to_string())?;
 
-        let local = EntityTransform::Translate(-base);
+        let local = EntityTransform::Translate((-base).as_dvec3());
         for mut entity in entities {
             view::dispatch::apply_transform(&mut entity, &local);
             entity = crate::modules::draw::modify::explode::normalize_entity_for_block(entity);