fix(annotative): start new drawings at the unit-aware annotation scale

A new drawing is metric with metre insertion units, but its annotation
multiplier stayed at Scene::new()'s unitless 1 while opened files and
CANNOSCALE convert paper millimetres into drawing units (0.001 at 1:1).
Annotative text and dimensions were drawn 1000x too large until the
first scale change, then shrank to their real size and looked gone,
also after switching back to 1:1.

One helper now derives the multiplier from the header; new drawings,
file open and every INSUNITS change (UNITS, the units popup, DWGUNITS,
SETVAR) go through it.

Fixes #1674

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/scene/entity.rs

@@ -3090,6 +3090,10 @@ impl Scene {
         // The name alone leaves the header pointing at the previous style, and
         // a DWG writes the current style by handle only.
         self.document.header.current_dimstyle_handle = iso;
+        // The annotation multiplier depends on those units; Scene::new()
+        // left it at the unitless 1, which drew annotative objects 1000x too
+        // large until the first CANNOSCALE change shrank them. (#1674)
+        self.sync_annotation_scale_from_header();
         for obj in self.document.objects.values_mut() {
             if let codec::objects::ObjectType::Layout(l) = obj {
                 if l.name != "Model" {