feat(plugin): harden V4 host integration

Preserve legacy wire indices and route concurrent V4 responses and background requests by correlation and stable tab ID.

src/app/update/file.rs

@@ -1154,13 +1154,20 @@ pub(super) fn on_open_file(&mut self) -> Task<Message> {
                     crate::scene::text::ttf_glyph::clear_fallback_cache();
                 }
                 // Current model-space annotation scale comes from the drawing's
-                // CANNOSCALEVALUE (paper/drawing factor); the multiplier we use
-                // for text/dim sizing is its inverse (1:50 -> 0.02 -> 50.0).
+                // CANNOSCALEVALUE (paper/drawing factor). Convert its inverse into
+                // drawing units as well: metric annotation sizes are paper millimetres
+                // and imperial annotation sizes are paper inches.
+                // Current model-space annotation scale comes from the drawing's
+                // CANNOSCALEVALUE (paper/drawing factor). Convert its paper unit into
+                // the drawing's INSUNITS as well, so e.g. a metre drawing uses
+                // 0.001 model units for 1 mm of paper at 1:1.
                 let cannoscale_value = self.tabs[i].scene.document.header.annotation_scale_value;
+                let unit_factor = self.tabs[i].scene.annotation_scale_unit_factor();
+
                 self.tabs[i].scene.annotation_scale = if cannoscale_value > 1e-9 {
-                    (1.0 / cannoscale_value) as f32
+                    ((1.0 / cannoscale_value) / unit_factor) as f32
                 } else {
-                    1.0
+                    (1.0 / unit_factor) as f32
                 };
 
                 // Open-time breakdown so regressions are visible immediately.