M6.10: Boolean operations hold their area identities on almost-horizontal edges

The sweep compares a curve with another segment over a y-span shorter than
its tolerance by bounding boxes only, so an edge rising by less than that
tolerance (e.g. a rectangle edge cut through a circle's cap, whose computed
intersection end sits ~1e-12 off the horizontal) was mis-ordered and the
result lost a corner: xor/difference/area disagreed by up to 10%. Every
sweep input now snaps on-curve y values within the tolerance to one level
(handles follow and are levelled with their anchor), so such edges become
exact horizontals, which the sweep handles exactly.

- tests/regressions.rs: the six minimal failures found by the properties,
  the root cause in isolation, and the self-crossing "star" polygon.
- The star-polygon strategy rejected no angular step of half a turn or
  more, so it sometimes drew self-crossing polygons (the shoelace failure).
- Property suites run from a fixed seed (PROPTEST_CASES / PROPTEST_RNG_SEED
  still override), so CI cannot fail at random.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/testkit/src/strategies.rs

@@ -19,9 +19,14 @@ pub fn arb_rect() -> impl Strategy<Value = Rect> {
 }
 
 /// A simple (non self-intersecting) star-shaped polygon: points at increasing angles around a
-/// centre with random radii.
+/// centre with random radii. Every angular step stays below half a turn: a wider one puts the
+/// centre outside and can make the polygon cross itself.
 pub fn arb_star_polygon() -> impl Strategy<Value = Vec<Point>> {
-    (arb_point(40.0, 160.0), prop::collection::vec((0.2f64..1.0, 5.0f64..40.0), 3..12)).prop_map(|(c, v)| {
+    let steps = prop::collection::vec((0.2f64..1.0, 5.0f64..40.0), 3..12).prop_filter("an angular step of half a turn or more", |v| {
+        let total: f64 = v.iter().map(|(a, _)| a).sum();
+        v.iter().all(|(a, _)| 2.0 * a < total)
+    });
+    (arb_point(40.0, 160.0), steps).prop_map(|(c, v)| {
         let total: f64 = v.iter().map(|(a, _)| a).sum();
         let mut ang = 0.0;
         v.iter()