Add cross timeline interaction gym

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

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+# ============================================================
+# Cross-Timeline Interactions Gym — Hub
+# ============================================================
+# Starting point for exploring all cross-timeline patterns.
+# Pick a pattern to see it in action, then return here.
+# ============================================================
+
+join Miko center
+join Advisor right
+
+Advisor (pl5): Welcome to the Cross-Timeline Interactions Gym!
+Miko (joy): Here you can explore all the ways Dialogic timelines\
+can talk to each other.
+
+label menu
+
+Miko (neutral): Which pattern would you like to explore?
+
+- 1. Jump to another timeline
+    Miko (smile): Simple! Let's see a direct jump in action.
+    jump gym_pattern1_jump/
+
+- 2. Jump + Return (subroutine)
+    Miko (joy): Like calling a function, but with dialog!
+    jump gym_pattern2_caller/start
+
+- 3. Variable chain (set in A, check in B)
+    Miko (neutral): Let's set some variables and see them carry over.
+    set {Gym.demo_flag} = false
+    jump gym_pattern3_variable_chain_A/start
+
+- 4. Signal → GDScript → Timeline
+    Miko (smile): Code listens for a signal and starts the next timeline.
+    [wait time="0.5"]
+    jump gym_pattern4_signal_chain/start
+
+- 5. Choice routes to different timelines
+    Miko (joy): Each choice jumps to a completely different scene!
+    jump gym_pattern5_choice_jump/start
+
+- 6. Condition-based routing
+    Miko (doubt): Variables decide which timeline runs, not the player.
+    jump gym_pattern6_condition_router/start
+
+- 7. Player text input drives the next timeline
+    Miko (smile): Type your name and see what happens!
+    jump gym_pattern7_text_input/start
+
+- 8. Dynamic label jump with variables
+    Miko (joy): The variable itself is the jump target!
+    jump gym_pattern8_dynamic_label/start
+
+- I've seen enough. Goodbye!
+    Miko (smile): Thanks for exploring! Come back anytime.
+    Advisor (pl5): Until next time.
+    [end_timeline]