feat(street-level): place coverage on the bare earth on Google 3D

Draped ground polylines and clamped billboards land on the top of the
Google Photorealistic 3D mesh, so on a street under trees or an overpass
the coverage seemed to float at roof height.

At street zoom on Google 3D (camera below 1,400 m, back above 1,800 m)
the core switches to a `terrain` surface mode. groundCast.js samples the
existing terrain service on the ground-floor grid (0.001 deg), interpolates
bilinearly and adds 2 m; Mapillary lines become plain polylines at those
heights, and cones and the position marker stand on the street, so the
mesh hides what is behind buildings and trees. A tile draws draped first
and swaps to its cast lines once the heights arrive, keeping the old lines
until the new primitives are ready. Lookups are once per tile and aborted
when the tile is dropped; a geoid fallback counts as unknown and leaves
lines draped. Other map stacks and higher views are unchanged.

Providers get getSurface() and groundCaster in their context and an
optional setSurface(mode); getUIState() exposes `surface`. The QA gate
checks draped on Esri and terrain on Google 3D.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

scripts/package-boundaries.json

@@ -1130,6 +1130,7 @@
       "src/layers/streetLevel/marker.js",
       "src/layers/streetLevel/view.js",
       "src/layers/streetLevel/uiState.js",
+      "src/layers/streetLevel/groundCast.js",
       "src/layers/streetLevel/pickRouter.js",
       "src/layers/streetLevel/params.js",
       "src/layers/streetLevel/registry.js",
@@ -2020,6 +2021,7 @@
       "src/layers/streetLevel/marker.js",
       "src/layers/streetLevel/view.js",
       "src/layers/streetLevel/uiState.js",
+      "src/layers/streetLevel/groundCast.js",
       "src/layers/streetLevel/pickRouter.js",
       "src/layers/streetLevel/params.js",
       "src/layers/streetLevel/registry.js",
@@ -2505,6 +2507,7 @@
       "src/layers/streetLevel/marker.js",
       "src/layers/streetLevel/view.js",
       "src/layers/streetLevel/uiState.js",
+      "src/layers/streetLevel/groundCast.js",
       "src/layers/streetLevel/pickRouter.js",
       "src/layers/streetLevel/params.js",
       "src/layers/streetLevel/registry.js",