feat(BitmapLayer): add tile loading placeholders

[charlieforward9]

Summary

Approaches another degree of feature parity with  / Google Maps.

Screen.Recording.2026-06-09.at.7.17.07.PM.mov

• add an opt-in TileLayer.renderPlaceholder hook for selected loading tiles with no loaded content
• keep cached refinement behavior by default, while allowing refinementStrategy: 'no-overlap' to prefer placeholders over cached ancestor/child content
• forward renderPlaceholder through tiled TerrainLayer
• document the placeholder API and add delayed-loading placeholder demos to the map-tile and image-tile examples
• add focused TileLayer and TerrainLayer coverage for placeholder rendering, refinement interaction, loading state, and prop forwarding

Caveat

This PR intentionally leaves one follow-up open: eliminate the intermediate frame between the placeholder and the final bitmap. The current API is scoped to rendering placeholders while tile content is absent; the placeholder-to-BitmapLayer transition can be tightened further in a follow-up.

Validation

• git diff --check
• npx prettier --config .prettierrc --check modules/geo-layers/src/tile-layer/tile-layer.ts modules/geo-layers/src/terrain-layer/terrain-layer.ts test/modules/geo-layers/tile-layer/tile-layer.spec.ts test/modules/geo-layers/terrain-layer.spec.ts examples/website/map-tile/app.tsx examples/website/image-tile/app.tsx
• npx eslint --no-error-on-unmatched-pattern modules/geo-layers/src/tile-layer/tile-layer.ts modules/geo-layers/src/terrain-layer/terrain-layer.ts test/modules/geo-layers/tile-layer/tile-layer.spec.ts test/modules/geo-layers/terrain-layer.spec.ts examples/website/map-tile/app.tsx examples/website/image-tile/app.tsx (0 errors; test files are ignored by the repo lint config)
• yarn test --project node test/modules/geo-layers/tile-layer/tile-layer.spec.ts test/modules/geo-layers/terrain-layer.spec.ts (2 files, 12 tests)

[chrisgervang]

Why are the paths not always visible on the globe? Is it a gap of tiles where no tiles are being requested, or are they there but covered by something?

In other words, is the grey we're seeing on the globe the back side or the front side?

Another approach to consider: Could this effect be implemented with an always-present grid layer that's just rendered underneath a tile layer? It's less clever, but also easier to reason about

[felixpalmer]

an always-present grid layer

I'm with @chrisgervang on this, it seems like an simpler way to achieve the same thing. The fact that there is a "caveat" which that adds a random blank grey frame doesn't smell good to me. I would say try to implement the result you want without modifying TileLayer first - the tile loading mechanics are complex enough already, I don't think we should be changing them unless necessary