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 <noreply@anthropic.com>

crates/pathops/src/offset.rs

@@ -90,7 +90,7 @@ pub fn offset_path(path: &PathData, delta: f64, join: Join, miter_limit: f64) ->
     } else {
         BezPath::new()
     };
-    let Ok(arr) = Arrangement::new(&[(fill, FillRule::NonZero), (ring, FillRule::NonZero), (open_bp, FillRule::NonZero)]) else {
+    let Ok(arr) = Arrangement::new(vec![(fill, FillRule::NonZero), (ring, FillRule::NonZero), (open_bp, FillRule::NonZero)]) else {
         return PathData::default();
     };
     let c = if delta >= 0.0 { arr.contours(|m| m[0] || m[1] || m[2]) } else { arr.contours(|m| m[0] && !m[1]) };