feat: add FER validation and CLI for density evolution

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

model/test_density_evolution.py

@@ -175,3 +175,27 @@ class TestPEGBaseMatrixConstructor:
         target = [7, 3, 3, 3, 2, 2, 2, 2]
         H_base, girth = construct_base_matrix(target, z=32, n_trials=500)
         assert girth >= 4, f"Girth {girth} should be >= 4"
+
+
+class TestFERValidationAndCLI:
+    """Tests for FER validation and CLI."""
+
+    def test_validate_returns_results(self):
+        """validate_matrix should return FER results dict."""
+        from density_evolution import validate_matrix
+        from ldpc_sim import H_BASE
+        np.random.seed(42)
+        results = validate_matrix(H_BASE, lam_s_points=[10.0], n_frames=10, lam_b=0.1)
+        assert 10.0 in results, f"Expected key 10.0, got {list(results.keys())}"
+        assert 'fer' in results[10.0]
+        assert 0.0 <= results[10.0]['fer'] <= 1.0
+
+    def test_cli_threshold(self):
+        """CLI threshold subcommand should exit 0."""
+        import subprocess
+        result = subprocess.run(
+            ['python3', 'model/density_evolution.py', 'threshold', '--z-pop', '5000', '--tol', '1.0'],
+            capture_output=True, text=True, timeout=120,
+        )
+        assert result.returncode == 0, f"CLI failed: {result.stderr}"
+        assert 'threshold' in result.stdout.lower() or 'photon' in result.stdout.lower()