Add EDDM size, income, businesses and USPS map link (#17)

## Summary
Closes #17.
- Join USPS EDDM GIS stats onto the carrier-route picker (household size, median income, business count) and link to the official EDDM map.
- GIS miss stays a dash and does not block Find routes; PCM households/cost unchanged.

site/messaging/README.md

@@ -120,6 +120,11 @@ Route mode calls PCM `POST /list/count/zipcode` (or `/radius`) with
 or `PCM_POSTCARD_PRICE_FIRSTCLASS` / `PCM_POSTCARD_PRICE_STANDARD`. Default
 purchased list type is `IRL` (`PCM_LIST_TYPE`). This is not official EDDM postage.
 
+Compose also joins USPS EDDM GIS stats (businesses; household size / median
+income when USPS fills them) and links to
+[the EDDM map](https://eddm.usps.com/eddm/select-routes.htm) for the official
+route table.
+
 | Extra env | Purpose |
 |-----------|---------|
 | `PCM_POSTCARD_PRICE_FIRSTCLASS` | Fallback $/piece if PCM omits FirstClass pricing |