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 <noreply@anthropic.com>

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 {

src/scene/mod.rs

@@ -1452,6 +1452,10 @@ impl NavPerfOp {
 #[derive(Clone, Copy, Debug)]
 pub(in crate::scene) struct NavPerfSample {
     pub(in crate::scene) op: NavPerfOp,
+    /// Which message caused it. An edit whose handler runs in under a
+    /// millisecond never reaches the `update` line's threshold, so without this
+    /// the log shows the latency but not what asked for it.
+    pub(in crate::scene) cause: &'static str,
     pub(in crate::scene) space: &'static str,
     pub(in crate::scene) mode: &'static str,
     pub(in crate::scene) started: iced::time::Instant,
@@ -2880,6 +2884,15 @@ impl Scene {
     }
 
     pub(crate) fn record_nav_perf(&self, op: NavPerfOp, started: iced::time::Instant) {
+        self.record_nav_perf_caused(op, "-", started);
+    }
+
+    pub(crate) fn record_nav_perf_caused(
+        &self,
+        op: NavPerfOp,
+        cause: &'static str,
+        started: iced::time::Instant,
+    ) {
         if !crate::perf::enabled() {
             return;
         }
@@ -2892,6 +2905,7 @@ impl Scene {
         };
         self.nav_perf_pending.set(Some(NavPerfSample {
             op,
+            cause,
             space,
             mode,
             started,

src/scene/view/render.rs

@@ -1216,8 +1216,9 @@ impl shader::Primitive for Primitive {
         let prepare_ms = nav_prepare_started.elapsed().as_secs_f64() * 1000.0;
         if let Some(sample) = self.nav_perf {
             crate::perf::record(format_args!(
-                "[perf] nav-prepare op={} space={} mode={} input={:.2}ms build={:.2}ms prepare={:.2}ms elapsed={:.2}ms viewports={}",
+                "[perf] nav-prepare op={} cause={} space={} mode={} input={:.2}ms build={:.2}ms prepare={:.2}ms elapsed={:.2}ms viewports={}",
                 sample.op.label(),
+                sample.cause,
                 sample.space,
                 sample.mode,
                 sample.input_ms,