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-bubble/registry-item.json

@@ -0,0 +1,20 @@
+{
+  "$schema": "https://hyperframes.heygen.com/schema/registry-item.json",
+  "name": "us-map-bubble",
+  "type": "hyperframes:block",
+  "title": "US Bubble Map",
+  "description": "Animated US bubble map with proportional city markers, value callouts, and connection lines — composable with us-map",
+  "tags": ["data", "map", "geography", "usa", "bubble", "cities"],
+  "dimensions": {
+    "width": 1920,
+    "height": 1080
+  },
+  "duration": 12,
+  "files": [
+    {
+      "path": "us-map-bubble.html",
+      "target": "compositions/us-map-bubble.html",
+      "type": "hyperframes:composition"
+    }
+  ]
+}