feat(ColumnLayer): add cap prop for dome and cone top geometry

[charlieforward9]

Summary

Adds a geometry-only cap prop to ColumnLayer that shapes the top of each extruded column. No new accessors, no shader changes, no per-instance attributes — zero performance impact on existing usage.

Value	Description
'flat' (default)	Current behavior, unchanged
'dome'	Smooth hemispherical cap with normals curving from outward to upward
'cone'	Pointed conical cap tapering to a single apex

Cap height equals the column radius. Only applies when extruded: true.

new ColumnLayer({
  data,
  getPosition: d => d.coords,
  getElevation: d => d.height,
  cap: 'dome',   // or 'cone' | 'flat'
  extruded: true
})

Why this belongs in the base layer

• Zero cost when unused — cap: 'flat' (the default) produces the exact same geometry as before. The cap is built at geometry-construction time, not per-instance.
• Generic applicability — dome/cone tops are useful anywhere extruded columns appear: 3D bar charts wanting softer tops, terrain/elevation vis, environmental data, architectural previews. This was also requested in 2020.
• Composable — since HexagonLayer renders ColumnLayer under the hood, this flows through to aggregation-based hexagon maps.
• Minimal surface area — one prop, three string values. Follows @felixpalmer's recommendation to keep scope limited to geometry modification.

Demo

agintel.ai use case for allowing users to swap between 3D TreeLayer and '2D' ColumnLayer - preserving aesthetic depth and visibility of tree (in contrast to the flat Scatterplot layer that can be a little more difficult to contrast and style.

Screen.Recording.2026-04-14.142120.mp4

Changes

• ColumnGeometry: When cap is not 'flat' and extruded: true, replaces the flat top tessellation with interleaved ring triangle strips. Dome uses max(2, round(diskResolution / 4)) rings along a quarter-circle arc; cone uses 1 ring to a single apex.
• ColumnLayer: Added cap prop (default 'flat'), forwarded through getGeometry → ColumnGeometry, wired into updateState geometry-regeneration check.
• Tests: Vertex counts, apex position, side geometry preservation, and height=0 passthrough.
• Docs: Added cap entry to column-layer.md.

Fixes #9942

Test plan

• Unit tests for cone/dome vertex counts, apex positions, side geometry preservation
• cap: 'flat' produces identical geometry to current behavior
• cap ignored when height=0 (not extruded)
• Visual verification (screen recording above)