@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/geometry/diffraction.py

@@ -0,0 +1,40 @@
+"""
+Knife-edge diffraction geometry calculations.
+"""
+
+import math
+
+
+def knife_edge_loss(
+    d1_m: float,
+    d2_m: float,
+    h_m: float,
+    wavelength_m: float,
+) -> float:
+    """
+    Calculate diffraction loss over single knife edge.
+
+    Args:
+        d1_m: Distance TX to obstacle
+        d2_m: Distance obstacle to RX
+        h_m: Obstacle height above LOS (positive = above)
+        wavelength_m: Signal wavelength
+
+    Returns:
+        Loss in dB (>= 0)
+    """
+    if d1_m <= 0 or d2_m <= 0 or wavelength_m <= 0:
+        return 0.0
+
+    v = h_m * math.sqrt(2 * (d1_m + d2_m) / (wavelength_m * d1_m * d2_m))
+
+    if v < -0.78:
+        L = 0.0
+    elif v < 0:
+        L = 6.02 + 9.11 * v - 1.27 * v ** 2
+    elif v < 2.4:
+        L = 6.02 + 9.11 * v + 1.65 * v ** 2
+    else:
+        L = 12.95 + 20 * math.log10(v)
+
+    return max(0.0, L)