Add cross timeline interaction gym

docs/gyms/cross-timeline-interactions/gym_pattern3_variable_chain_B.dtl

@@ -0,0 +1,30 @@
+# ============================================================
+# Pattern 3: Variable Chain — Timeline B (checks the variable)
+# ============================================================
+
+label start
+
+join Advisor right
+
+Advisor (pl5): I'm timeline B. Let me check the variable you set...
+
+[wait time="1.0"]
+
+if {Gym.demo_flag}:
+    Advisor (surprise): It's TRUE! Timeline A successfully set it.
+    Advisor (pl5): This proves that variables persist across timeline\
+    boundaries — jumps, returns, even `start_timeline()` calls.
+else:
+    Advisor (doubt): It's false... something went wrong.
+    Advisor (doubt): Did you skip timeline A?
+
+[wait time="1.5"]
+
+Advisor (pl5): Key insight: variables are shared GLOBAL state.
+Advisor (doubt): Any timeline can read or write any pre-declared variable.
+Advisor (pl5): This makes them perfect for world state, quest flags,\
+relationship scores, and inventory.
+
+[wait time="1.0"]
+
+jump gym_main/menu