feat: simplify range-daymark to concentric triangles; preserve land cache

map.js — _encRangeDaymarkCanvas:
  Remove tripod legs, hub ring and mast. Symbol is now two clean
  concentric triangles pointing up: outer WHITE + inner BLACK,
  matching the standard IALA dayboard. Optional light flare at apex
  if the feature has LITCHR. No extra structures.

chart_manager.py — _build_cache:
  Don't overwrite existing land/hazard/zone GeoJSON files with an
  empty result when the .000 has no LNDARE/OBSTRN/ZONBND layer.
  Preserves hand-built or legacy cache data for custom charts
  (e.g. Barranquilla built from nav-aids-only CSV).

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

backend/services/chart_manager.py

@@ -805,18 +805,27 @@ def _build_cache(cell_id: str, enc_path: Path):
     log.info("ENC %s → %d depth features", cell_id, len(depths))
 
     land = _parse_land(enc_path)
-    with open(cell_dir / "land.geojson", "w") as f:
-        json.dump({"type": "FeatureCollection", "features": land}, f)
-    log.info("ENC %s → %d land features", cell_id, len(land))
+    land_path = cell_dir / "land.geojson"
+    # Preserve existing land cache when the .000 has no LNDARE layer
+    # (e.g. custom charts built from CSV that only contain nav aids)
+    if land or not land_path.exists():
+        with open(land_path, "w") as f:
+            json.dump({"type": "FeatureCollection", "features": land}, f)
+    log.info("ENC %s → %d land features%s", cell_id, len(land),
+             " (preserved existing)" if not land and land_path.exists() else "")
 
     hazards = _parse_hazards(enc_path)
-    with open(cell_dir / "hazards.geojson", "w") as f:
-        json.dump({"type": "FeatureCollection", "features": hazards}, f)
+    hazards_path = cell_dir / "hazards.geojson"
+    if hazards or not hazards_path.exists():
+        with open(hazards_path, "w") as f:
+            json.dump({"type": "FeatureCollection", "features": hazards}, f)
     log.info("ENC %s → %d hazard features", cell_id, len(hazards))
 
     zones = _parse_zones(enc_path)
-    with open(cell_dir / "zones.geojson", "w") as f:
-        json.dump({"type": "FeatureCollection", "features": zones}, f)
+    zones_path = cell_dir / "zones.geojson"
+    if zones or not zones_path.exists():
+        with open(zones_path, "w") as f:
+            json.dump({"type": "FeatureCollection", "features": zones}, f)
     log.info("ENC %s → %d zone features", cell_id, len(zones))
 
     # Cache count and bbox in meta.json so list_cells() doesn't need to read features.geojson