HeatmapLayer: accept borrowed GPUVector inputs - #11061

Note

Low Risk
Low risk as this introduces an opt-in GPU vector path on WebGPU without modifying the default accessor and attribute pipelines.

Overview
Adds support for borrowed positions and weights GPUVector props in HeatmapLayer when running on WebGPU. This allows callers to bypass CPU accessors and supply pre-allocated GPU column buffers directly while preserving byte offsets and strides.

The layer derives instance counts from vector lengths or numInstances prefixes, configuring explicit buffer layouts and managing zeroed low-precision position buffers for weight-transform aggregation. When GPU vector props are omitted, standard attribute updates and accessor pathways remain unchanged.

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