perf: measure widget-tree construction

Everything measured so far accounts for about 1 ms of a ~120 ms pan
frame: update is 5-28 ms at worst and usually far less, prepare 0.45 ms,
encode 0.5 ms. Meanwhile the GPU idles at 0-8%, encoding no scene at all
leaves latency unchanged, major page faults stay flat at 17 across forty
seconds of panning, and freeing 6 GB of swap changes nothing either.

So the time is in code that has never been timed. `view()` runs between
the input handler returning and `prepare` being called, rebuilds the
whole widget tree every frame, and is the largest unmeasured span in
that gap.

Reports only when it exceeds 1 ms, so a healthy frame stays quiet.

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

src/app/view/mod.rs

@@ -192,7 +192,20 @@ impl OpenCADStudio {
         // All dialogs are in-canvas modals now (Plan B); view_main stacks the
         // active one. `window_id` is unused — there is only the main window.
         let _ = window_id;
-        self.view_main()
+        // Pan latency on a large drawing is ~120 ms while `update`, `prepare`
+        // and `encode` together account for ~1 ms of it, the GPU sits at 0-8%,
+        // and encoding no scene at all changes nothing. Widget-tree
+        // construction is the largest span between the input handler returning
+        // and `prepare` running, and it was never measured.
+        let started = crate::perf::enabled().then(iced::time::Instant::now);
+        let element = self.view_main();
+        if let Some(started) = started {
+            let elapsed_ms = started.elapsed().as_secs_f64() * 1000.0;
+            if elapsed_ms >= 1.0 {
+                crate::perf_record!("[perf] view {:>7.1}ms", elapsed_ms);
+            }
+        }
+        element
     }
 
     /// The primary window: viewport, ribbon, tab bar, status bar. Split out of