Add advanced radar engineering models: clutter, propagation, and CFAR

Implement three new radar submodules for realistic system-level trade studies:

- clutter.py: Sea (GIT model), ground (Nathanson), and rain volume clutter
  models with SCR/SCNR calculations and resolution cell geometry

- propagation.py: Atmospheric attenuation (ITU-R), rain attenuation (P.838),
  radar horizon with 4/3 Earth model, and multipath fading

- cfar.py: CA-CFAR, OS-CFAR, GO-CFAR, SO-CFAR threshold calculations
  with associated detection loss models

Extended RadarDetectionScenario with clutter, propagation, and CFAR parameters.
Integrated all models into RadarModel.evaluate() returning new metrics.
Added 42 new unit tests (76 total radar tests, 263 total package tests pass).

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

Author: jman4162

src/phased_array_systems/models/radar/__init__.py

@@ -1,5 +1,27 @@
 """Radar detection models."""
 
+from phased_array_systems.models.radar.cfar import (
+    ca_cfar_threshold_factor,
+    cfar_loss_db,
+    cfar_threshold_factor,
+    compute_pd_with_cfar,
+    go_cfar_threshold_factor,
+    optimal_reference_cells,
+    os_cfar_threshold_factor,
+    so_cfar_threshold_factor,
+)
+from phased_array_systems.models.radar.clutter import (
+    compute_resolution_cell_area,
+    compute_resolution_volume,
+    compute_scnr,
+    compute_scr,
+    ground_clutter_rcs,
+    ground_clutter_sigma0,
+    rain_clutter_rcs,
+    rain_reflectivity,
+    sea_clutter_rcs,
+    sea_clutter_sigma0,
+)
 from phased_array_systems.models.radar.detection import (
     albersheim_snr,
     compute_detection_threshold,
@@ -12,14 +34,54 @@
     integration_loss,
     noncoherent_integration_gain,
 )
+from phased_array_systems.models.radar.propagation import (
+    atmospheric_attenuation_db_per_km,
+    atmospheric_loss_db,
+    grazing_angle_deg,
+    multipath_fading_factor,
+    radar_horizon_km,
+    rain_attenuation_db,
+    rain_attenuation_rate,
+)
 
 __all__ = [
+    # Main model
     "RadarModel",
+    # Detection
     "compute_detection_threshold",
     "compute_pd_from_snr",
     "compute_snr_for_pd",
     "albersheim_snr",
+    # Integration
     "coherent_integration_gain",
     "noncoherent_integration_gain",
     "integration_loss",
+    # Clutter
+    "sea_clutter_sigma0",
+    "sea_clutter_rcs",
+    "ground_clutter_sigma0",
+    "ground_clutter_rcs",
+    "rain_reflectivity",
+    "rain_clutter_rcs",
+    "compute_resolution_cell_area",
+    "compute_resolution_volume",
+    "compute_scr",
+    "compute_scnr",
+    # Propagation
+    "atmospheric_attenuation_db_per_km",
+    "atmospheric_loss_db",
+    "rain_attenuation_rate",
+    "rain_attenuation_db",
+    "radar_horizon_km",
+    "grazing_angle_deg",
+    "multipath_fading_factor",
+    # CFAR
+    "ca_cfar_threshold_factor",
+    "os_cfar_threshold_factor",
+    "go_cfar_threshold_factor",
+    "so_cfar_threshold_factor",
+    "cfar_threshold_factor",
+    "cfar_loss_db",
+    "optimal_reference_cells",
+    "compute_pd_with_cfar",
 ]