feat: add Z=128 support for matrix construction and validation

Make validate_matrix() and run_full_pipeline() accept z parameter
instead of using hardcoded Z=32. Thread cn_mode/alpha to validation.
Add --z/--cn-mode/--alpha CLI options to full pipeline.

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

model/test_density_evolution.py

@@ -283,6 +283,31 @@ class TestNormalizedMinSumDE:
         )
 
 
+class TestZ128Support:
+    """Tests for Z=128 matrix construction and optimization."""
+
+    def test_construct_z128_valid(self):
+        """Construct matrix for [7,4,4,4,4,3,3,3] with z=128. Verify full rank, girth >= 6, encodable."""
+        from density_evolution import construct_base_matrix, verify_matrix
+        np.random.seed(42)
+        target = [7, 4, 4, 4, 4, 3, 3, 3]
+        H_base, girth = construct_base_matrix(target, z=128, n_trials=500)
+        checks = verify_matrix(H_base, z=128)
+        assert checks['full_rank'], f"Full rank failed: {checks['actual_rank']}/{checks['expected_rank']}"
+        assert girth >= 6, f"Girth {girth} should be >= 6"
+        assert checks['encodable'], "Should be encodable"
+
+    def test_validate_z128_runs(self):
+        """Run validate_matrix with z=128, n_frames=5 at lam_s=5. Verify returns dict with FER."""
+        from density_evolution import construct_base_matrix, validate_matrix
+        np.random.seed(42)
+        target = [7, 4, 4, 4, 4, 3, 3, 3]
+        H_base, girth = construct_base_matrix(target, z=128, n_trials=200)
+        results = validate_matrix(H_base, lam_s_points=[5.0], n_frames=5, lam_b=0.1, z=128)
+        assert 5.0 in results, f"Expected key 5.0, got {list(results.keys())}"
+        assert 'fer' in results[5.0]
+
+
 class TestAlphaOptimization:
     """Tests for alpha sweep optimization."""