Add cross timeline interaction gym

docs/gyms/cross-timeline-interactions/gym_pattern2_subroutine.dtl

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+# ============================================================
+# Pattern 2 Subroutine: The "Called" Timeline
+# ============================================================
+# This is the subroutine — a reusable dialog chunk that does
+# some work and then returns to wherever it was called from.
+# ============================================================
+
+label start
+
+leave Miko
+leave Advisor
+
+join Merchant left
+
+Merchant (happy): I'm the subroutine timeline!
+Merchant (blink): I can do work independently and then return.
+
+[wait time="1.0"]
+
+Merchant (happy): First, let me set a variable...
+set {Gym.sub_value} = 42
+
+Merchant (happy): Variable `Gym.sub_value` is now 42.
+Merchant (blink): The calling timeline will see this change!
+
+[wait time="1.0"]
+
+Merchant (surprise): Now watch this — I'll call `return`...
+
+# This pops the jump stack and resumes the caller
+# right after the `jump` that called us
+return
+
+# Nothing below here will execute
+Merchant (happy): You should never see this text!