Summary

Introduces the IntoBezPathsTolerance / IntoBezPaths traits and dedicated implementations for multi-geometry types, laying the groundwork for correct multi-polygon hatching in #188.

Problem

When a geo::MultiPolygon (e.g. from boolean operations) is converted to a single BezPath, all rings are flattened together and structural boundaries between polygons are lost. The hatching code then uses the heuristic "first subpath = exterior, rest = holes", which misinterprets the second polygon's exterior as a hole in the first.

Solution

This PR adds the new trait infrastructure. The next PR (#188) wires it into Draw::add_path.

• New traits: IntoBezPathsTolerance and IntoBezPaths (plural), which return Vec<BezPath> — one per structural unit.
• Blanket impl: Any type implementing IntoBezPathTolerance (singular) automatically gets IntoBezPathsTolerance returning a one-element Vec, so all existing single-geometry types (kurbo shapes, geo::Polygon, etc.) work unchanged.
• Dedicated impls for multi-geometry types (MultiPolygon, MultiLineString, MultiPoint, GeometryCollection, Geometry) that return one BezPath per constituent, preserving structural boundaries. The old IntoBezPathTolerance impls for these types are removed (they now get it via the blanket from their dedicated IntoBezPathsTolerance impl).
• Tests verifying that MultiPolygon yields multiple bezpaths and single types yield one.

Files changed

This is part 5 of 6 in a stack made with GitButler:

•  6  Add support for shape filling to whiskers #188
•  5  Introduce IntoBezPaths trait #191 👈
•  4  Add support for layer/path metadata inheritance #187
•  3  Add hatching algorithm to Path and FlattenedPath #186
•  2  Add .buffer() to Path and FlattenedPath #184
•  1  Add low-level implementation for path joining and merging optimization #183