feat(registry): add maps section with 4 new map blocks + docs

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).

registry/blocks/us-map-hex/registry-item.json

@@ -0,0 +1,17 @@
+{
+  "$schema": "https://hyperframes.heygen.com/schema/registry-item.json",
+  "name": "us-map-hex",
+  "type": "hyperframes:block",
+  "title": "US Hex Grid Map",
+  "description": "Animated hexagonal tile grid map — each state as an equal-weight hex with data fill and abbreviation label",
+  "tags": ["data", "map", "geography", "usa", "hexgrid", "tilegrid"],
+  "dimensions": { "width": 1920, "height": 1080 },
+  "duration": 10,
+  "files": [
+    {
+      "path": "us-map-hex.html",
+      "target": "compositions/us-map-hex.html",
+      "type": "hyperframes:composition"
+    }
+  ]
+}