bench(navigation): add viewport navigation and spatial candidate benchmarks

- Add Section 19 benchmarks covering camera pan viewport uniform updates,
  spatial hover candidate search, and viewport data assembly scaling
- Add navigation telemetry probes in src/perf.rs (active only when
  PERF=1) to track frame interval, batched input moves, and layout-draw gap

src/app/update/mod.rs

@@ -530,7 +530,9 @@ impl OpenCADStudio {
             }
         }
         if let Some(started) = perf_started {
-            let elapsed_ms = started.elapsed().as_secs_f64() * 1000.0;
+            let elapsed_us = started.elapsed().as_micros() as u64;
+            crate::perf::note_update_time(elapsed_us);
+            let elapsed_ms = elapsed_us as f64 / 1000.0;
             if elapsed_ms >= 5.0 {
                 crate::perf_record!("[perf] update {:>7.1}ms message={perf_label}", elapsed_ms,);
             }

src/app/update/viewport.rs

@@ -1201,6 +1201,7 @@ impl OpenCADStudio {
     }
 
     pub(in crate::app) fn on_viewport_move(&mut self, p: Point) -> Task<Message> {
+        crate::perf::note_pointer_move();
         // A ribbon dropdown is open over the viewport. Its backdrop
         // cannot swallow cursor motion — in iced 0.14 mouse_area/opaque
         // capture only button presses, never CursorMoved — so the move
@@ -5683,6 +5684,7 @@ properties={:.1}ms picked={}",
     pub(super) fn on_viewport_scroll(&mut self, delta: mouse::ScrollDelta) -> Task<Message> {
         match scroll_intent(delta, self.touchpad_pan, self.ctrl_down) {
             ScrollIntent::Zoom { notches } => {
+                crate::perf::note_scroll_zoom();
                 let mut s =
                     notches * crate::app::settings::zoom_notch_steps(self.zoom_factor);
                 if self.zoom_wheel_reversed {

src/app/view/mod.rs

@@ -202,6 +202,7 @@ impl OpenCADStudio {
                 crate::perf_record!("[perf] view {:>7.1}ms", elapsed_ms);
             }
         }
+        crate::perf::note_view_finish();
         element
     }