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

.mam.env.example

@@ -128,6 +128,20 @@
 #default: 3
 # SKS_EMPTY_GIVEUP=3
 
+# Minimum columns a pane must retain after a vertical split (2xK layout engine).
+#default: 60
+# MAM_MIN_PANE_COLS=60
+
+# Minimum rows a pane must retain after a horizontal split (2xK layout engine).
+#default: 20
+# MAM_MIN_PANE_ROWS=20
+
+# 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
+
 # ==============================================================================
 # deploy / distribution source (for forks/mirrors)
 # ==============================================================================