RFC(geo-layers) Share TerrainLayer tile data

[ibgreen-openai]

This is a stacked PR on top of the SharedTileLayer PR #10367

Summary

• Add an experimental _TerrainSource plus TerrainLayer._terrainTileset path so multiple TerrainLayer instances can reuse one loaded terrain tile payload cache through _SharedTileset2D.
• Keep the existing TerrainLayer URL-template API intact. The shared path is caller-owned and opt-in; elevationData still uses the existing internal TileLayer path when _terrainTileset is not supplied.
• Load projection-independent normalized terrain payloads once, then cache derived MapView/GlobeView render meshes on the shared tile data as each terrain layer needs them.
• Add the small _SharedTileset2D hooks needed for terrain payload byte accounting, plus focused tests and docs.

Why

TerrainLayer currently bakes viewport-projected bounds into each loaded tiled mesh, so separate terrain layers fetch and materialize the same source tile data independently. This stacks on #10367 and uses the shared tileset/cache machinery instead of adding a terrain-specific parallel cache.

Example

import {
  TerrainLayer,
  _SharedTileset2D,
  _TerrainSource,
  type _TerrainTileData as TerrainTileData
} from '@deck.gl/geo-layers';

const terrainSource = new _TerrainSource({...});

const terrainTileset = new _SharedTileset2D<TerrainTileData>({
  getTileData: tile => terrainSource.getTileData(tile)
});

new TerrainLayer({
  id: 'terrain-a',
  _terrainTileset: terrainTileset,
  ...
});

new TerrainLayer({
  id: 'terrain-b',
  _terrainTileset: terrainTileset,
  ...
});

The source owns elevation/texture/decoder/load configuration, the shared tileset owns tile traversal and cache state, and each TerrainLayer still owns its per-layer render props such as color, wireframe, material, and operation.

Validation

• yarn build
• yarn lint (passes with existing repo warnings only)
• yarn vitest run --project headless test/modules/geo-layers/terrain-layer-loading.spec.ts
• yarn vitest run --project node test/modules/imports.node.spec.ts
• yarn test-headless was also run; the relevant terrain/shared specs passed, but the full suite still has an unrelated reproducible LoadingWidget failure and one DeckGL#mount/unmount failure that passed when rerun in isolation.

[coveralls]

Coverage is 82.933% — ib/shared-terrain into ib/shared-tile-layer. No base build found for ib/shared-tile-layer.