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,);
             }