@mytec: feat: Phase 3.0 Architecture Refactor ✅

Major refactoring of RFCP backend:
- Modular propagation models (8 models)
- SharedMemoryManager for terrain data
- ProcessPoolExecutor parallel processing
- WebSocket progress streaming
- Building filtering pipeline (351k → 15k)
- 82 unit tests

Performance: Standard preset 38s → 5s (7.6x speedup)

Known issue: Detailed preset timeout (fix in 3.1.0)

backend/app/services/dominant_path_service.py

@@ -139,12 +139,33 @@ def find_dominant_paths_vectorized(
     """
     global _vec_log_count
 
+    # Fast path: no buildings at all → direct LOS, skip all numpy work
+    has_spatial_data = spatial_idx is not None and spatial_idx._grid
+    if not buildings and not has_spatial_data:
+        return {
+            'has_los': True,
+            'path_type': 'direct',
+            'total_loss': 0.0,
+            'path_length': 0.0,
+            'reflection_point': None,
+        }
+
     # Get nearby buildings via spatial index (same filtering as sync version)
     if spatial_idx:
         line_buildings = spatial_idx.query_line(tx_lat, tx_lon, rx_lat, rx_lon)
     else:
         line_buildings = buildings
 
+    # No nearby buildings along this line → direct LOS
+    if not line_buildings:
+        return {
+            'has_los': True,
+            'path_type': 'direct',
+            'total_loss': 0.0,
+            'path_length': 0.0,
+            'reflection_point': None,
+        }
+
     line_buildings = _filter_buildings_by_distance(
         line_buildings,
         (tx_lat, tx_lon), (rx_lat, rx_lon),
@@ -654,6 +675,19 @@ class DominantPathService:
             buildings: fallback list (only used if spatial_idx is None)
             spatial_idx: grid-based spatial index for fast local queries
         """
+        # Fast path: no buildings at all → direct LOS only
+        has_spatial_data = spatial_idx is not None and spatial_idx._grid
+        if not buildings and not has_spatial_data:
+            distance = terrain_service.haversine_distance(tx_lat, tx_lon, rx_lat, rx_lon)
+            return [RayPath(
+                path_type="direct",
+                total_distance=distance,
+                path_loss=self._calculate_path_loss(distance, frequency_mhz, tx_height, rx_height),
+                reflection_points=[],
+                materials_crossed=[],
+                is_valid=True,
+            )]
+
         paths = []
 
         # Use spatial index to get only buildings along the TX→RX line