Add cross timeline interaction gym

docs/gyms/cross-timeline-interactions/gym_pattern4_signal_target.dtl

@@ -0,0 +1,34 @@
+# ============================================================
+# Pattern 4 Target: Started by GDScript Signal Handler
+# ============================================================
+# This timeline was started by gym_controller.gd in response
+# to the "demo_chain" signal from gym_pattern4_signal_chain.dtl.
+# ============================================================
+
+join Advisor right
+
+Advisor (surprise): I was started by GDScript!
+Advisor (pl5): The `dialogic.signal_event` signal fired with\
+the argument "demo_chain".
+
+[wait time="1.0"]
+
+Advisor (pl5): The controller script caught it and called\
+`Dialogic.start_timeline("gym_pattern4_signal_target")`.
+
+Advisor (doubt): This is the most flexible pattern because...
+Advisor (pl5): ...GDScript can do ANYTHING before starting the next timeline:\
+load scenes, animate sprites, play audio, check conditions...
+
+[wait time="2.0"]
+
+Advisor (pl5): Signal args can be strings (like "demo_chain")\
+or full dictionaries for complex data.
+
+Advisor (surprise): One caveat: `start_timeline()` doesn't use the jump stack.
+Advisor (pl5): So there's no automatic `return` path. You control flow in code.
+
+[wait time="1.5"]
+
+Advisor (pl5): Ready to go back to the hub?
+jump gym_main/menu