First commit of claude's rework in django + vanillajs fronted

apps/sessions/ballistics.py

@@ -0,0 +1,32 @@
+"""
+Ballistic correction engine.
+
+Computes scope elevation and windage corrections given a rig, ammo,
+target distance, and weather conditions.
+
+Currently a stub — returns None until the trajectory integration is built.
+"""
+
+
+def compute_corrections(session, stage) -> dict:
+    """
+    Return scope corrections for a given PRS stage.
+
+    Args:
+        session: PRSSession instance (provides rig, ammo/reloaded_batch, weather)
+        stage:   PRSStage instance (provides distance_m)
+
+    Returns:
+        dict with keys: elevation, windage, unit, message
+    """
+    # TODO: implement point-mass trajectory integration using:
+    #   - session.rig.zero_distance_m, session.rig.scope_height_mm
+    #   - ammo BC (Bullet.bc_g7 / bc_g1) and muzzle velocity
+    #   - session weather fields (temperature_c, pressure_hpa, humidity_pct)
+    #   - stage.distance_m and session wind fields
+    return {
+        'elevation': None,
+        'windage': None,
+        'unit': None,
+        'message': 'Ballistic engine not yet implemented.',
+    }