GSOC 26: custom vertex attributes in multi-material buildGeometry

[Nixxx19]

follow up to #9063.

before, buildGeometry() only exposed the per-material parts when there were multiple materials and no custom per-vertex attributes. if you used vertexProperty(), the split was skipped and the geometry stayed whole, so you lost the material parts.

this makes custom vertex attributes work with the multi-material split:

• the per-part builder now carries each draw's custom attributes into its part, reindexed per part, with the same missing-value padding the combined geometry already does
• GeometryPart got a vertexProperty() method for it, and the prop factory is now shared with p5.Geometry instead of duplicated
• dropped the gate in finish() that kept the geometry whole when custom attributes were present

added a test that builds a two material geometry with a custom attribute and checks each part carries its own values aligned to its vertices. the change is in the shared builder so it works in both webgl and webgpu.

[p5-bot]

Continuous Release

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Commit hash: cf3cab7

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