Add another gym about scene switching

docs/gyms/gym09_scene_switching/README.md

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+# Gym 09 — Scene Switching
+
+Demonstrates switching between a Dialogic timeline scene and a non-Dialogic gameplay scene.
+
+## Flow
+
+```
+gym09_main.tscn          gym09_minigame.tscn
+┌─────────────────┐      ┌──────────────────┐
+│ gym09_intro.dtl │      │ Press button 5x   │
+│      ↓          │      │      ↓            │
+│ [signal]        │ ───→ │ set variable      │
+│                 │ ←─── │ change_scene back │
+│ gym09_congrats  │      │                  │
+│     .dtl        │      └──────────────────┘
+└─────────────────┘
+```
+
+## How to Run
+
+Open `gym09_main.tscn` in the Godot editor and press **F6** (Run Current Scene).
+
+## Key Mechanic
+
+- `Dialogic.VAR.Gym.minigame_done` acts as a flag between scenes
+- `[signal arg=start_minigame]` triggers `get_tree().change_scene_to_file()`
+- The controller's `_ready()` checks the flag to know which timeline to start