feat(statusbar): selection cycling for overlapping objects (SC pill)

Add a click_hits_all hit-test and an SC pill that, when on, makes
repeated clicks at the same spot step through the overlapping objects
there (nearest first) instead of accumulating the selection. The whole
cycling path is gated behind the toggle, so default picking is unchanged
when it is off.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/hit_test.rs

@@ -55,6 +55,43 @@ pub fn click_hit<'a>(
     best
 }
 
+/// Like `click_hit` but returns every wire within the click threshold,
+/// nearest first. Used by selection cycling to step through overlapping
+/// objects under the cursor.
+pub fn click_hits_all<'a>(
+    cursor: Point,
+    wires: &'a [WireModel],
+    view_proj: Mat4,
+    bounds: Rectangle,
+) -> Vec<&'a str> {
+    let mut hits: Vec<(f32, &str)> = Vec::new();
+    for wire in wires {
+        let mut prev: Option<Point> = None;
+        let mut best_for_wire = CLICK_THRESHOLD_PX;
+        let mut hit = false;
+        for &[px, py, pz] in &wire.points {
+            if px.is_nan() {
+                prev = None;
+                continue;
+            }
+            let cur = world_to_screen(Vec3::new(px, py, pz), view_proj, bounds);
+            if let Some(p0) = prev {
+                let d = dist_point_to_segment(cursor, p0, cur);
+                if d < best_for_wire {
+                    best_for_wire = d;
+                    hit = true;
+                }
+            }
+            prev = Some(cur);
+        }
+        if hit {
+            hits.push((best_for_wire, &wire.name));
+        }
+    }
+    hits.sort_by(|a, b| a.0.partial_cmp(&b.0).unwrap_or(std::cmp::Ordering::Equal));
+    hits.into_iter().map(|(_, name)| name).collect()
+}
+
 /// Pick a 3D solid by clicking anywhere on its shaded body: project each
 /// mesh triangle to screen space and test whether the cursor lands inside it.
 /// Returns the front-most hit (smallest projected depth). Lets meshed solids