Keep a zoomed-in profile's hover fill: count the slabs each edge crosses (#336)

fill_triangles bounded the slab triangulator by edges x slabs, as if every
edge crossed every slab. A ring sampled finely at a close zoom has hundreds
of edges and slabs but only four edges in each slab, so the estimate ran a
hundred times over the hover highlight's budget and the fill was dropped,
leaving only the outline. Count the slabs each edge actually spans instead.

Region2::triangulate tested every edge against every slab, so its work was
the overestimate. It now sweeps upward with the edges crossing the current
slab, giving the same triangles in time proportional to them.

Fixes #164

Co-authored-by: Vitali M. <6042958+vitalibr@users.noreply.github.com>

crates/ui-egui/src/sketch_display_ui_tests.rs

@@ -289,3 +289,37 @@ fn pixel_frame_budget_does_not_cache_unbounded_empty_curve_results() {
     assert!(h.app.viewport.display.lock().unwrap().len() <= 4096);
     assert_eq!(h.app.session.doc.to_json(), baseline);
 }
+
+/// A zoomed-in ring is sampled finely, but each slab of the fill still crosses only its
+/// four boundary edges: its fill must fit the hover highlight's budget (#164).
+#[test]
+fn zoomed_in_ring_fill_fits_the_hover_highlight_budget() {
+    let mut h = Harness::new();
+    h.app.ui.show_origin = false;
+    h.app.ui.perspective = true;
+    h.app.run("sketch.create", json!({"plane":"XY","name":"Ring"})).unwrap();
+    h.app.run("sketch.circle.center", json!({"center":[0,0],"radius":40})).unwrap();
+    h.app.run("sketch.circle.center", json!({"center":[0,0],"radius":45})).unwrap();
+    h.settle();
+    h.step(&json!({"call":"ui.view","params":{"view":"iso"}})).unwrap();
+    h.step(&json!({"call":"ui.view","params":{"view":"fit"}})).unwrap();
+    h.app.cam.zoom_at(0.15, Some(Vec3::new(42.5, 0.0, 0.0)));
+    h.settle();
+    let baseline = h.app.session.doc.to_json();
+    let sid = h.app.session.active_sketch.unwrap();
+    let state = h.app.session.world_state();
+    let ss = state.sketch(sid).unwrap();
+    let proj = projection(&h.app, h.app.viewport.rect.unwrap());
+    let ppp = h.app.viewport.pixels_per_point();
+    let ring = ss.profiles.iter().find(|p| p.region.holes.len() == 1).unwrap();
+    let display = crate::sketch_display::display_region(&ring.region, &ss.plane, &proj, ppp).unwrap();
+    let edges = std::iter::once(&display.outer).chain(&display.holes).map(|l| l.segs.len()).sum::<usize>();
+    assert!(edges * edges / 2 > 65_536, "fixture must sample the ring finely enough to exceed the old estimate: {edges} edges");
+    let triangles = crate::sketch_display::region_triangles(&ring.region, &ss.plane, &proj, ppp);
+    assert!(!triangles.is_empty(), "the ring's fill was dropped at this zoom");
+    for t in &triangles {
+        let r = ((t[0] + t[1] + t[2]) / 3.0).len();
+        assert!((39.9..=45.1).contains(&r), "fill triangle outside the ring: r = {r}");
+    }
+    assert_eq!(h.app.session.doc.to_json(), baseline);
+}