feat(layout): implement deterministic 2xK grid engine with single decision table

- Change N=1->2 split policy from down to right to establish 2 full-height columns
- Share single decision table across GUI and headless modes
- Add _full_height_pane guard to prevent half-height column splits
- Quadruple-wire default max_columns=2 and max_rows=2 across layout.py, lib.sh, and .mam.env.example
- Update unit and integration tests to enforce deterministic 2x2 grid trajectory

tests/test_b19_headless_reconcile_fixes.py

@@ -28,11 +28,16 @@ def test_bug2_headless_layout_does_not_overflow():
     assert not d_wide.is_overflow
     assert d_wide.direction == "right"
 
-    # 4. Tall pane (split down)
+    # 4. Tall-but-narrow pane: N=1 is always right; 80//2 < min_cols 60 → overflow
     payload_tall = {"result": {"panes": [{"pane_id": "p1", "rect": {"width": 80, "height": 60}}]}}
     d_tall = compute_2xk_layout(payload_tall, min_cols=60, min_rows=20)
-    assert not d_tall.is_overflow
-    assert d_tall.direction == "down"
+    assert d_tall.is_overflow
+    assert d_tall.direction == "overflow"
+
+    payload_tall_wide = {"result": {"panes": [{"pane_id": "p1", "rect": {"width": 160, "height": 60}}]}}
+    d_tall_wide = compute_2xk_layout(payload_tall_wide, min_cols=60, min_rows=20)
+    assert not d_tall_wide.is_overflow
+    assert d_tall_wide.direction == "right"
 
 
 def test_bug3_reconcile_skills_dir_passed_and_fallback():