Add low-level implementation for path joining and merging optimization

[abey79]

This PR adds support for joining to paths, and path merging optimisation to layers. This is only the low-level machinery. It is not exposed yet to either the CLI or whiskers.

Constants

• EPSILON: f64 = 1e-10 — point coincidence threshold

PathDataTrait

• is_closed(&self) -> bool — checks if start ≈ end

PathTrait

• join(&mut self, other: &Self, epsilon: f64) — append path, merge at coincident endpoints
• split(self) -> Vec<Self> — split compound paths at MoveTo boundaries

PathMetadata

• merge(&mut self, other: &PathMetadata) — merge metadata (first wins)

LayerTrait

• join_paths(&mut self, tolerance: f64, flip: bool) — join adjacent paths
• explode(&mut self) — split all compound paths into subpaths

Free Functions (vsvg::optimization)

• sort_paths<P, D>(paths: &mut Vec<P>, flip: bool)
• sort_paths_with_builder<P, D>(paths: &mut Vec<P>, builder: IndexBuilder)
• join_paths<P, D>(paths: &mut Vec<P>, tolerance: f64, flip: bool)

Changed

• LayerTrait::sort/sort_with_builder now delegate to free functions

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This is part 1 of 4 in a stack made with GitButler:

•  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 👈