Add BLM roads/trails layer

- Add BLM routes vector source from pmtiles
- Style: dashed olive/sage lines, distinct from USFS
- Labels at zoom 12+ using ROUTE_PRMRY_NM
- Hit-area layer (14px) for click targets
- Popup showing route name, asset class, transport, surface, length
- "BLM Roads" toggle in Layer Control panel
- Feature-flag gated: has_blm_trails (default false)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/config.js

@@ -34,6 +34,7 @@ const FALLBACK_CONFIG = {
     has_contours_test_10ft: false,
     has_address_book_write: false,
     has_usfs_trails: false,
+    has_blm_trails: false,
     has_contacts: false,
   },
   defaults: {