Phase 2/3/5 of the building-blockage support:
* Parser streams csv.gz tiles into compact records (centroid, radius,
height) — keeps RAM ~32 B per polygon, drops -1.0-height entries.
* Index buckets records into 0.01° (~1 km) grid cells in a public ETS
table for concurrent reads; max_height_near/3 and records_near/3
scan only the 9 nearest buckets.
* Loader lazily parses any cached quadkey before a Calculate so the
scorer is terrain-only when tiles aren't on disk yet.
* Rover.PathTerrain now treats each path-sample obstacle as
terrain_elev + max_local_building_height, so blocked LOS through
recent construction actually reduces clearance.
* Selecting a candidate pushes a candidate_buildings event with the
buildings near each rover→station path; the hook renders them as
height-coloured circles (yellow <10 m, orange 10-30 m, red >=30 m)
with a tooltip showing height in meters.
Phase 1 of building-blockage support: implements quadkey math (Bing/MS
tile encoding), looks up the MS dataset-links index for UnitedStates,
and downloads csv.gz quadkey tiles to /data/buildings (overridable via
:buildings_cache_dir). Index is cached 24h; tiles are written once and
reused. No path-analysis wiring yet — that's the next slice.
