New blocks: world-map (D3 Natural Earth choropleth), us-map-bubble
(proportional city markers), us-map-hex (hexagonal tile grid),
us-map-flow (animated connection arcs between cities).
All blocks share the same dark theme and are composable — layer
bubble or flow on top of the choropleth via track indexes.
Adds "Maps" section to the catalog docs with MDX pages for all 5
map blocks (including the us-map from the previous commit).