feat(layout,test): resolve backlog items I-2, I-3, and C-1 with unanimous peer review

- I-2: add 5.0s upper-bound execution time assertion in test_bug4_headless_unobservable_fast_path to contractually guard SKS_EMPTY_GIVEUP early-exit latency
- I-3: clean up PaneInfo.focused, wire MAM_MAX_PANE_COLS/MAM_MAX_COLS environment variables and CLI --max-cols flag
- C-1: apply max_columns growth guard to headless 0x0 layouts, mirroring GUI behavior
- Promoted Planner consensus plan Rev.2 (plan-fea5f1b2.md) and unanimous PASS reports from Reviewers Claude (report-55d1a1d9.md) and Cline (report-6f18ba0f.md)
- Verified all 333 test cases pass with exit code 0

tests/test_layout.py

@@ -6,9 +6,7 @@ import pytest
 
 from lib_py.layout import (
     compute_2xk_layout,
-    extract_panes_and_focus,
     LayoutDecision,
-    PaneInfo,
 )
 
 
@@ -336,3 +334,76 @@ echo "SHIM_OK"
     assert "SHIM_OK" in res.stdout
 
 
+def _four_panes_two_columns():
+    """GUI payload: 2 full columns x 2 rows (4 panes). Shared by the max-cols tests."""
+    return {
+        "result": {
+            "panes": [
+                {"pane_id": "p1", "rect": {"x": 0, "y": 0, "width": 100, "height": 40}},
+                {"pane_id": "p2", "rect": {"x": 0, "y": 40, "width": 100, "height": 40}},
+                {"pane_id": "p3", "rect": {"x": 100, "y": 0, "width": 100, "height": 40}},
+                {"pane_id": "p4", "rect": {"x": 100, "y": 40, "width": 100, "height": 40}},
+            ]
+        }
+    }
+
+
+def test_cli_max_cols_flag_triggers_overflow():
+    """CLI --max-cols reaches compute_2xk_layout (the lib.sh-facing path)."""
+    payload = json.dumps(_four_panes_two_columns())
+    skills_dir = os.path.abspath(".agents/skills")
+    env = {**os.environ, "PYTHONPATH": skills_dir}
+    res = subprocess.run(
+        [sys.executable, "-m", "lib_py.layout",
+         "--min-cols", "30", "--min-rows", "20", "--max-cols", "2", "--json"],
+        input=payload, capture_output=True, text=True, env=env)
+    assert res.returncode == 0, res.stderr
+    d = json.loads(res.stdout)
+    assert d["direction"] == "overflow" and d["is_overflow"]
+    assert d["reason"] == "max_columns_reached"
+
+
+def test_env_max_cols_applies_without_flag():
+    """MAM_MAX_PANE_COLS is honoured with no --max-cols flag, which is exactly
+    how lib.sh invokes the module (lib.sh passes no --max-cols)."""
+    payload = json.dumps(_four_panes_two_columns())
+    skills_dir = os.path.abspath(".agents/skills")
+    env = {**os.environ, "PYTHONPATH": skills_dir, "MAM_MAX_PANE_COLS": "2"}
+    res = subprocess.run(
+        [sys.executable, "-m", "lib_py.layout",
+         "--min-cols", "30", "--min-rows", "20", "--json"],
+        input=payload, capture_output=True, text=True, env=env)
+    assert res.returncode == 0, res.stderr
+    assert json.loads(res.stdout)["reason"] == "max_columns_reached"
+
+
+def test_headless_max_columns_growth_guard():
+    """C-1: headless mode must honour max_columns too.
+
+    A headless 2xK grid completes n // 2 columns, so at n=4 with max_columns=2
+    a further `right` split would open a third column and must overflow instead.
+    Note the cap blocks *opening* a new column; it does not force an existing
+    over-cap layout to shrink -- the odd-n `down` branch (and the GUI's
+    fill_singleton_column) deliberately ignore it.
+    """
+    def headless(n):
+        return {"result": {"panes": [
+            {"pane_id": f"p{i}", "rect": {"x": 0, "y": 0, "width": 0, "height": 0}}
+            for i in range(1, n + 1)]}}
+
+    d4 = compute_2xk_layout(headless(4), max_columns=2)
+    assert d4.is_overflow and d4.direction == "overflow"
+    assert d4.reason == "max_columns_reached"
+
+    # Continues growing below the cap
+    d2 = compute_2xk_layout(headless(2), max_columns=2)
+    assert d2.direction == "right" and not d2.is_overflow
+
+    # Filling an existing column is not blocked (mirrors GUI fill_singleton_column)
+    d3 = compute_2xk_layout(headless(3), max_columns=2)
+    assert d3.direction == "down" and not d3.is_overflow
+
+    # When max_columns is not set, existing alternation is preserved (behavior neutrality)
+    assert compute_2xk_layout(headless(4)).direction == "right"
+
+