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.
For each (rover cell, fixed station) pair, sample SRTM along the
great-circle path and add a per-link dB bonus proportional to the
rover's elevation above the highest intermediate terrain. Rover spots
on hilltops with clear sight to multiple stations now rise to the top.
Also vendor Leaflet's layers control PNGs and copy them under
priv/static/assets/css/images/ on build so /rover stops 404'ing on
layers-2x.png.
