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

.mam.env.example

@@ -144,8 +144,13 @@
 # Maximum number of columns a workspace may grow to before the engine reports
 # 'overflow' (which makes lib.sh create a fresh workspace instead of splitting).
 # Applies to both measured (GUI) and headless 0x0 layouts.
-#default: (unset -> no column cap)
-# MAM_MAX_PANE_COLS=3
+#default: 2
+# MAM_MAX_PANE_COLS=2
+
+# Maximum number of rows per column before overflow. Default 2 guarantees an
+# even 2x2 grid; do not raise this until a resize-normalisation pass exists.
+#default: 2
+# MAM_MAX_PANE_ROWS=2
 
 # ==============================================================================
 # deploy / distribution source (for forks/mirrors)