feat: Row index based picking

[ibgreen-openai]

row-index based picking

• This is PR 2 of 3
• Follow-up to 1st PR feat(core): Remove redundant instance picking colors #10275
• Future 3rd PR: switch to the luma.gl indexPicking module, which will remove the now shader-internal use of picking colors as well as lift restrictions on number of layers and instances in picking.

Summary

This PR replaces remaining explicit color-encoded picking buffers with semantic rowIndexes uint32 buffers.

All custom picking attributes are now called rowIndexes and contain canonical uint32 row/feature ids rather than obscure picking colors.

Instead of storing pre-encoded RGB picking colors in pickingColors / instancePickingColors, layers that need custom vertex/instance to row mappings now store the logical object or feature id directly in rowIndexes, and the shader derives the picking color with picking_getPickingColorFromIndex(rowIndexes).

Why

rowIndexes are a more natural representation than encoded color bytes:

• they directly express the logical row/object/feature id used for picking
• they avoid CPU-side RGB color encoding for explicit picking buffers
• they use one uint32 value instead of RGB/RGBA byte tuples
• they are useful beyond picking, for example as indexes into storage buffers or metadata tables

Built-in one-instance-per-object layers still use gl_InstanceID / instance_index. Explicit rowIndexes are only used where the logical picking id differs from the rendered instance id.

What Changed

• Added rowIndexes as the unified explicit picking-index attribute.
• Converted special built-in layers from explicit picking color buffers to rowIndexes.
• Updated GLSL/WGSL shaders to derive picking colors from row indexes.
• Kept legacy explicit pickingColors / instancePickingColors support in core for compatibility.
• Updated binary GeoJSON picking helpers from color arrays to uint32 index arrays.
• Updated custom layer docs and upgrade notes to recommend rowIndexes.

Layer Cases

Layer / Path	rowIndexes Represents
PathLayer	source path row for each generated segment/joint instance
MultiIconLayer	source text row for each generated character instance
SolidPolygonLayer	source polygon row, including __source.index for composite data
Binary GeoJSON/MVT points	global feature id for rendered point geometry
Binary GeoJSON/MVT lines	global feature id for rendered path geometry
Binary GeoJSON/MVT polygons	global feature id for rendered polygon geometry
Binary GeoJSON/MVT polygon outlines	global feature id for rendered outline geometry
IconLayer / ScatterplotLayer binary hooks	caller-provided logical picking index per instance
MeshLayer feature-id picking	feature id for each mesh vertex

Testing

• npx tsc -p modules/core/tsconfig.json --noEmit
• npx tsc -p modules/layers/tsconfig.json --noEmit
• `npx t

[coveralls]

coverage: 83.384% (-0.03%) from 83.416% — ib/row-index-based-picking into master