refactor(scene): remove the world_offset field (P7)

Delete the world_offset field from Scene and DerivedCaches and the
load-time assignment; every reader now uses a zero origin. Geometry is
absolute and the double-single relative-to-eye pipeline keeps it precise,
so the coarse origin is gone. world_offset_from_centers still derives
local_extent_max (camera/cull span) but returns no offset. The remaining
`world_offset: [f64;3]` function parameters are now always zero — dead
plumbing left for a later mechanical sweep; behaviour is unchanged.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/app/mtext_editor.rs

@@ -395,7 +395,7 @@ impl super::OpenCADStudio {
         let Some(ed) = self.mtext_editor.as_ref() else { return };
         let mt = ed.build_mtext();
         let entity = EntityType::MText(mt.clone());
-        let woff = self.tabs[i].scene.world_offset;
+        let woff = [0.0_f64; 3];
         let anno = self.tabs[i].scene.annotation_scale;
         let wires: Vec<WireModel> = tessellate::tessellate(
             &self.tabs[i].scene.document,