Add support for layer/path metadata inheritance

[abey79]

This PR refactors path metadata to support inheritance semantics, including:

• PathMetadata fields are now Option<T>: None means "inherit from layer" (or SVG default if layer also `None`)
• Added LayerMetadata::default_path_metadata: Layer-level defaults that paths can inherit from
• SVG export writes only non-None values: Layer defaults on <g>, path overrides on <path>, letting SVG handle inheritance
• SVG reader promotes common metadata: After loading, if all paths in a layer share the same value, it's moved to layer defaults and cleared from paths. This is needed because usvg flattens everything, and lead to duplication of layer-level metadata to all paths, leading to file size bloat.
• Added ResolvedPathMetadata: For display purposes (vsvg-viewer) where concrete values are needed
• Added PathMetadata::merge(): Conflict semantics (both Some but different → None)

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This is part 4 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