feat: add contours layer with tier-aware zoom visibility

- Adds contour PMTiles vector source (contours-na.pmtiles)
- Minor/intermediate/index tier rendering at z11+/z8+/z4+
- Elevation labels on index contours at z12+
- Dark theme opacity adjustment
- has_contours feature flag gated

Completes T pipeline integration (Phase 1).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/config.js

@@ -29,6 +29,7 @@ const FALLBACK_CONFIG = {
     has_traffic_overlay: false,
     has_landclass: false,
     has_public_lands_layer: false,
+    has_contours: true,
     has_address_book_write: false,
     has_contacts: false,
   },