perf: say which message caused an edit's latency

The first measurement showed four consecutive edits at ~192 ms each with
a handler cost of 0.55 ms. That is below the `update` line's 5 ms
threshold, so the log reports the latency but not what asked for it —
and whether a live command preview is bumping the geometry epoch on every
mouse move is exactly the question the numbers raise.

`nav-prepare` now carries the message label alongside the op.

Co-Authored-By: Claude Opus 5 <[email protected]>

src/app/update/mod.rs

@@ -334,8 +334,11 @@ impl OpenCADStudio {
         if let (Some(started), Some(before)) = (perf_started, perf_epoch_before) {
             let tab = &self.tabs[self.active_tab];
             if tab.scene.geometry_epoch != before {
-                tab.scene
-                    .record_nav_perf(crate::scene::NavPerfOp::Edit, started);
+                tab.scene.record_nav_perf_caused(
+                    crate::scene::NavPerfOp::Edit,
+                    perf_label,
+                    started,
+                );
             }
         }
         if let Some(started) = perf_started {