Cesium: keyless Terrarium terrain provider and COG-backed terrain

[giswqs]

Part of #2259. Cesium parity roadmap, tier 3.

Problem

CesiumCanvas only enables terrain when a Cesium Ion token is present (terrainEnabled && Boolean(ionToken)), because setTerrainEnabled calls createWorldTerrainAsync. getTerrainCogSource returns null, hasCustomTerrainSource is false, and setTerrainCogSource resolves false. So the Controls → Terrain toggle and the Terrain Settings dialog do nothing on a keyless globe, while the 2D map has keyless AWS Terrarium terrain plus COG-backed terrain.

Proposal

• Write a TerrariumTerrainProvider implementing Cesium's TerrainProvider: fetch the AWS Terrain Tiles PNG, decode RGB → metres, downsample to a HeightmapTerrainData grid (65×65 or 33×33), with a WebMercatorTilingScheme and availability derived from the source's max zoom. Cesium consumes heightmap terrain natively, so no quantized-mesh work is required.
• Point the same provider at the existing COG → Terrarium tile producer in cog-dem-source.ts to implement setTerrainCogSource, making the Terrain Settings dialog engine-neutral.
• Keep Cesium World Terrain as the upgrade when a token exists; drop the token gate in CesiumCanvas.
• Mount TerrainSettingsDialog on the globe (it is in the MapLibre-only subtree in DesktopShell).

Effort

Medium.

[giswqs]

Implemented in #2302: TerrariumTerrainProvider (packages/map/src/cesium-terrarium.ts) serves keyless AWS Terrarium heightmaps with shared tile edges and bounded requests; CesiumEngine.setTerrainCogSource points the same provider at the COG DEM tile producer so the Terrain Settings dialog is engine-neutral (it is mounted outside the MapLibre-only subtree in DesktopShell); Cesium World Terrain is still selected when an Ion token exists, and the token gate on the Controls → Terrain toggle is gone. Verified on main today. Closing.