perf: warm the interaction handle index during open

The first hover over the canvas after opening a large drawing froze the
UI for a second:

    hover-detail  query=0.2 handles=986.1 ... candidates=253
    hover-dwell   986.6ms

`interaction_handle_index` and the insert-hatch map it reads are built
lazily, and first use is that hover. Neither can move to the loader
thread: they read `hatches`, `meshes` and `block_meshes`, which are
installed on the real `Scene` only after the loader has finished.

Build them at the end of the open instead, where the user is already
waiting. This does not make the work cheaper -- open grows by roughly the
second the hover used to cost -- but a second inside an operation that is
already seconds long reads very differently from a second-long freeze on
a mouse move.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/app/update/file.rs

@@ -1438,6 +1438,8 @@ pub(super) fn on_open_file(&mut self) -> Task<Message> {
                 if let Some(prepared) = prepared_geometry {
                     self.tabs[i].scene.install_prepared_open_geometry(prepared);
                 }
+                // Pay for the interaction index here, not on the first hover.
+                self.tabs[i].scene.warm_interaction_caches();
                 self.tabs[i]
                     .scene
                     .replace_selection(rustc_hash::FxHashSet::default());