feat(layers): extend WebGPU layer support

[ibgreen-openai]

Goals

• Stack WebGPU layer support on top of #10151, which adds explicit WebGPU attribute buffer groups.
• Make the website examples that have WebGPU-capable base layers selectable through the WebGPU tab.
• Let WebGPU render supported base layers even when an application supplies WebGL-only layer extensions.
• Keep the WebGL path as close to master as possible.

Changes

• Added WGSL implementations and WebGPU-specific attribute/buffer paths for ArcLayer, ColumnLayer/GridCellLayer, PathLayer, SolidPolygonLayer/PolygonLayer/GeoJsonLayer, TextLayer, ScenegraphLayer, HexagonLayer's cell renderer, ScreenGridLayer's cell renderer, and BitmapLayer.
• Kept IconLayer and TextLayer's dense instance attributes in interleaved buffer groups to stay under WebGPU vertex-buffer binding pressure.
• HexagonLayer, ContourLayer, and ScreenGridLayer use their existing CPU aggregation paths on WebGPU, then render their sublayers with WGSL. Their WebGL GPU aggregation paths are unchanged.
• BitmapLayer's WGSL path enables the geospatial TileLayer and WMSLayer website examples.
• Fixed the shared WebGPU layer-pass blend state to use premultiplied-alpha over blending, correcting ArcLayer and HexagonLayer compositing without changing WebGL parameters.
• WebGPU vertex layouts now exclude index attributes while preserving the existing separate index-buffer binding.
• WebGPU now ignores LayerExtension shaders, attributes, and lifecycle/draw hooks; WebGL keeps the existing extension behavior.
• Kept WebGPU-only buffer layouts, attribute types, transition disables, and render parameters behind device.type === 'webgpu' so the WebGL path remains as close to master as possible.
• Added/updated the WebGPU support guide and wired website device tabs only for examples that passed a live WebGPU smoke test.

Website examples verified on WebGPU

Website example	Main layers exercised	WebGPU behavior
ArcLayer	GeoJsonLayer, ArcLayer	Renders flat arcs
BrushingExtension	ScatterplotLayer, ArcLayer	Renders; BrushingExtension is ignored
CollisionFilterExtension	GeoJsonLayer, TextLayer	Renders; CollisionFilterExtension is ignored
ContourLayer	ContourLayer, PathLayer, SolidPolygonLayer	CPU aggregation + WGSL sublayer rendering
DataFilterExtension	ScatterplotLayer	Renders; DataFilterExtension is ignored
GeoJsonLayer (Paths)	GeoJsonLayer, PathLayer	Renders paths
GeoJsonLayer (Polygons)	GeoJsonLayer, SolidPolygonLayer	Renders polygons
HexagonLayer	HexagonLayer, HexagonCellLayer	CPU aggregation + WGSL cell rendering
IconLayer	IconLayer	Renders icons
LineLayer	LineLayer	Renders lines
PointCloudLayer	PointCloudLayer	Renders points
ScatterplotLayer	ScatterplotLayer	Renders points
ScenegraphLayer	ScenegraphLayer	Renders glTF scenegraphs
ScreenGridLayer	ScreenGridLayer, ScreenGridCellLayer	CPU aggregation + WGSL cell rendering
TextLayer (Clipping)	TextLayer	Renders clipped text
TextLayer	TextLayer	Renders text; CollisionFilterExtension is ignored
TileLayer (Geospatial)	TileLayer, BitmapLayer	Renders raster tiles
WMSLayer	_WMSLayer, BitmapLayer	Renders WMS raster imagery

Validation

• yarn build
• yarn lint (passes with the repository's existing 70 module warnings and 18 example warnings)
• yarn test-website
• npx vitest run --project node test/modules/core/lib/attribute/attribute-buffer-groups.node.spec.ts (7/7)
• yarn test-headless test/modules/layers/bitmap-layer.spec.ts test/modules/aggregation-layers/screen-grid-layer.spec.ts test/modules/aggregation-layers/screengrid-cell-layer.spec.ts test/modules/aggregation-layers/contour-layer/contour-layer.spec.ts test/modules/aggregation-layers/hexagon-layer.spec.ts (11/11)
• yarn test-render test/render/test-cases/arc-layer.spec.ts test/render/test-cases/hexagon-layer.spec.ts test/render/test-cases/contour-layer.spec.ts test/render/test-cases/screen-grid-layer.spec.ts test/render/test-cases/path-layer.spec.ts test/render/test-cases/core-layers.spec.ts (39/39)
• Assembled and parsed the BitmapLayer and ScreenGridLayer WGSL after the final build.
• Live website WebGPU smoke sweep across every example in the table; no WebGPU shader, binding, or draw errors.
• Live WebGL smoke checks for representative changed examples; no errors.

Notes

• Extensions are intentionally no-ops on WebGPU in this PR; their visual/filtering behavior is not ported.
• Aggregation layers use CPU fallback on WebGPU; WebGL GPU aggregation remains unchanged.
• BitmapLayer is still documented as partial support: the geospatial TileLayer and WMSLayer examples pass, but the non-geospatial lunar Image Tile example still renders blank without a WebGPU error, so its toggle is intentionally omitted.
• ArcLayer's verified website path is the default flat-arc path; GreatCircleLayer remains outside this support set.
• ScenegraphLayer's external example asset can occasionally fail to fetch independently of WebGPU; its shader/render path was verified separately.

[coveralls]

Coverage is 83.034% — codex/webgpu-layer-stack into ib/buffer-groups. No base build found for ib/buffer-groups.