feat(extensions): TerrainExtension GlobeView support

[charlieforward9]

Summary

Makes TerrainExtension (draped rendering of layers onto a terrain surface) work on GlobeView as well as MapView.

The existing implementation packed layer bounds into the live viewport's common space, which on GlobeView is sphere cartesian — incomparable with screen-space bounds, and invalid as a UV basis for the draped sampler. Cover/height-map FBOs therefore rendered with corrupted UV math on globe.

All bounds are now expressed in absolute Web Mercator common space, regardless of the active projection. The FBO itself is rendered via a WebMercatorViewport in both cases, so the cover/height-map texture becomes projection-invariant — the same draw target can be sampled from MapView and GlobeView without re-rendering, and a projection toggle does not invalidate it.

Changes:

• projection-utils — new MERCATOR_REFERENCE_VIEWPORT plus helpers lngLatToMercatorCommon and getMercatorReferenceViewport so other modules can compute bounds in the shared absolute-Mercator basis.
• terrain-cover, height-map-builder — project layer bounds through the Mercator reference viewport. On globe, skip the viewport-bounds intersection in getRenderBounds (which would yield sphere cartesian coords) and use full layer bounds. makeViewport unprojects bounds through the Mercator reference instead of the live viewport for geospatial inputs.
• shader-module — compute terrainMercPos per-fragment by unprojecting globe cartesian back to lng/lat and forward-projecting through project_mercator_, so USE_HEIGHT_MAP / USE_COVER sample against the absolute-Mercator bounds uniform on any projection. The project module's mercator helpers are VS-only, so the value is passed via varying; terrain meshes are fine enough that interpolation error is negligible. The bounds uniform is packed as absolute Mercator (no commonOrigin subtract) since the shader computes absolute xy itself.

Example app (examples/website/terrain-extension) updated with a MapView/GlobeView toggle so the demo exercises both projections. (Requires TerrainLayer's tesselator: 'grid' from the companion geo-layers PR.)

Why

The docs list _TerrainExtension as experimental, but it's already widely used for draping icons/labels/polygons onto terrain. Extending it to GlobeView lets apps keep the same draped layers across projection toggles without needing a separate code path.

Test plan

• MapView with TerrainExtension and TerrainLayer source — unchanged behavior
• GlobeView with TerrainExtension and TerrainLayer(tesselator: 'grid') source — layers drape correctly on sphere
• Projection toggle without re-rendering the cover/height-map FBO
• USE_HEIGHT_MAP (elevation offsets for icons/text) correct on both projections
• USE_COVER (draped overlays on terrain surface) correct on both projections

Companion PRs

• Depends on feat(geo-layers): TerrainLayer GlobeView + grid tesselator #10247 for the example app to demonstrate the full stack. The extension changes themselves are independent and compile without that PR.
• feat(core): GlobeView pitch, bearing, and cursor-anchored zoom #10246 is not required but improves the demo UX.

[charlieforward9]

Superseded by #10251 (rebranched under cr/ prefix).