feat(extensions): add ColumnShapeExtension for per-instance bevel shapes

[charlieforward9]

Summary

Closes #9942

Adds a new ColumnShapeExtension to @deck.gl/extensions that enables per-instance dome, cone, and custom bevel shapes on ColumnLayer, with per-instance radius multipliers.

This is a non-invasive LayerExtension approach — it does not modify ColumnLayer internals. Instead it:

• Injects shader hooks (DECKGL_FILTER_SIZE, #main-end) for GPU-side vertex transformation
• Monkey-patches getGeometry to swap in dome-enabled geometry with concentric bevel rings and smooth normals
• Adds 4 instanced attributes (instanceBevelSegs, instanceBevelHeights, instanceBevelBulge, instanceRadii)

Features

• getBevel accessor: 'flat' | 'dome' | 'cone' | {segs, height, bulge} per instance
• getRadius accessor: per-instance radius multiplier
• Dome geometry with smooth spherical normals
• Bulge effect for convex/concave dome surfaces
• Correct Gouraud lighting recomputation for shaped caps

Use cases

• Tree canopy visualization (dome caps sized by canopy area)
• Architectural column rendering with styled tops
• 3D bar charts with per-bar cap styles

Example

import {ColumnLayer} from '@deck.gl/layers';
import {ColumnShapeExtension} from '@deck.gl/extensions';

new ColumnLayer({
  data: TREES,
  extruded: true,
  extensions: [new ColumnShapeExtension()],
  getBevel: d => ({segs: 8, height: d.canopyHeight, bulge: 0.2}),
  getRadius: d => d.canopyRadius,
});

Prior art

• feat(experimental/ColumnLayer) Trees deck.gl-community#470 (ExperimentalColumnLayer)
• feat(ColumnLayer): getBevel & getRadius #9933 (getBevel & getRadius proposal)

Test plan

• Unit tests added (test/modules/extensions/column-shape.spec.ts)
• API documentation added (docs/api-reference/extensions/column-shape-extension.md)
• Extension registered in @deck.gl/extensions index with type exports
• Visual regression tests (render test cases)
• Manual testing with tree canopy dataset

🤖 Generated with Claude Code

[charlieforward9]

@chrisgervang @felixpalmer I cannot add reviewers so tagging for visibility.