feat: add Monte Carlo density evolution engine

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

model/test_density_evolution.py

@@ -0,0 +1,52 @@
+#!/usr/bin/env python3
+"""Tests for density evolution optimizer."""
+
+import numpy as np
+import pytest
+import sys
+import os
+
+sys.path.insert(0, os.path.dirname(__file__))
+
+
+class TestDensityEvolution:
+    """Tests for the Monte Carlo DE engine."""
+
+    def test_de_known_good_converges(self):
+        """DE with original staircase profile at lam_s=10 should converge easily."""
+        from density_evolution import run_de, ORIGINAL_STAIRCASE_PROFILE
+        np.random.seed(42)
+        converged, error_frac = run_de(
+            ORIGINAL_STAIRCASE_PROFILE, lam_s=10.0, lam_b=0.1,
+            z_pop=10000, max_iter=50
+        )
+        assert converged, f"DE should converge at lam_s=10, error_frac={error_frac}"
+
+    def test_de_known_bad_fails(self):
+        """DE at very low lam_s=0.1 should not converge."""
+        from density_evolution import run_de, ORIGINAL_STAIRCASE_PROFILE
+        np.random.seed(42)
+        converged, error_frac = run_de(
+            ORIGINAL_STAIRCASE_PROFILE, lam_s=0.1, lam_b=0.1,
+            z_pop=10000, max_iter=50
+        )
+        assert not converged, f"DE should NOT converge at lam_s=0.1, error_frac={error_frac}"
+
+    def test_de_population_shape(self):
+        """Verify belief arrays have correct shapes after one step."""
+        from density_evolution import de_channel_init, density_evolution_step
+        np.random.seed(42)
+        n_base = 8
+        m_base = 7
+        z_pop = 1000
+
+        # Original staircase H_base profile
+        from density_evolution import ORIGINAL_STAIRCASE_PROFILE
+        beliefs, msg_memory = de_channel_init(ORIGINAL_STAIRCASE_PROFILE, z_pop, lam_s=5.0, lam_b=0.1)
+
+        # beliefs should be (n_base, z_pop)
+        assert beliefs.shape == (n_base, z_pop), f"Expected ({n_base}, {z_pop}), got {beliefs.shape}"
+
+        # Run one step
+        beliefs = density_evolution_step(beliefs, msg_memory, ORIGINAL_STAIRCASE_PROFILE, z_pop)
+        assert beliefs.shape == (n_base, z_pop), f"Shape changed after step: {beliefs.shape}"