Add another gym about scene switching

docs/gyms/gym09_scene_switching/gym09_intro.dtl

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+# ============================================================
+# Gym 09 — Scene Switching: Intro Timeline
+# ============================================================
+# This timeline introduces the scene-switching concept,
+# then emits a signal that triggers a scene change to the
+# minigame (a non-Dialogic scene).
+# ============================================================
+
+join Miko center
+join Advisor right
+
+Miko (joy): Welcome to Gym 09 — Scene Switching!
+Advisor (pl5): This gym demonstrates how Dialogic timelines and\
+non-Dialogic gameplay scenes can flow back and forth.
+
+[wait time="1.0"]
+
+Miko (neutral): Here's the plan:
+Miko (smile): 1. You're reading this dialog (Dialogic timeline).
+Advisor (pl5): 2. I'll emit a signal that changes the scene to a\
+pure minigame — no dialog, no portraits, just gameplay.
+Miko (smile): 3. In the minigame, press the button 5 times!
+Advisor (pl5): 4. After 5 presses, you'll automatically return here\
+and I'll congratulate you with another timeline.
+
+[wait time="1.5"]
+
+Miko (joy): That shows the full loop:\
+Dialogic scene → gameplay scene → Dialogic scene!
+
+Advisor (pl5): The key insight: Dialogic is an autoload, so\
+variables persist even in non-Dialogic scenes.
+
+[wait time="1.0"]
+
+Miko (smile): Ready? Let's go to the minigame!
+
+# This signal is caught by gym09_controller.gd,
+# which calls get_tree().change_scene_to_file("...minigame.tscn")
+[signal arg=start_minigame]
+
+# This line won't be reached — the scene changes immediately
+Miko (surprise): Wait, shouldn't I still be here?