Add PolygonShape.split_to_convex (polygon to convex tiles)

Ports geometry/planar/Polygon.java (corner de-duplication and collinear
removal, winding number) and PolygonShape.java, including the recursive
split_to_convex that decomposes a simple polygon into convex Simplex
tiles by dividing at concave corners along minimal axis-parallel lines.

Orientation and convexity tests are exact; the division-point search is
approximate (float line evaluation, split point rounded to an integer
corner) as upstream. The concave-corner search starts deterministically
at corner 0 rather than a seeded PRNG; this only affects which valid
decomposition is produced.

Supporting additions: Simplex.from_corners (convex polygon to simplex)
and Line.function_value_approx / function_in_y_value_approx.

Invariant tests over L, plus, staircase and square polygons: tile areas
sum to the polygon area (no gaps, no overlap), a point is in the polygon
iff in some tile, and no point is strictly inside more than one tile
(interiors disjoint). Also covers Polygon normalization and orientation.

src/freeroute/geometry/line.py

@@ -198,6 +198,28 @@ class Line:
             result = self.side_of(line_1.intersection(line_2))
         return result
 
+    def function_value_approx(self, x: float) -> float:
+        """Approximate ``y`` on this (non-vertical) line at abscissa ``x``."""
+        p1 = self.a.to_float()
+        p2 = self.b.to_float()
+        dx = p2.x - p1.x
+        if dx == 0:
+            return 0.0
+        dy = p2.y - p1.y
+        det = p1.x * p2.y - p2.x * p1.y
+        return (dy * x - det) / dx
+
+    def function_in_y_value_approx(self, y: float) -> float:
+        """Approximate ``x`` on this (non-horizontal) line at ordinate ``y``."""
+        p1 = self.a.to_float()
+        p2 = self.b.to_float()
+        dy = p2.y - p1.y
+        if dy == 0:
+            return 0.0
+        dx = p2.x - p1.x
+        det = p1.x * p2.y - p2.x * p1.y
+        return (dx * y + det) / dy
+
     # --- intersection (exact) ----------------------------------------------
 
     def intersection(self, other: Line) -> Point: