Create directory-structure

Add an initially planned project structure diagram

directory-structure

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+opentunesai/
+├── 🤖 Models
+│   ├── melody-master
+│   ├── visual-composer
+│   ├── sound-scenes
+│   └── music-canvas
+│
+├── 📚 Datasets
+│   ├── midi-collection
+│   ├── audio-samples
+│   ├── genre-specific
+│   └── multimodal-pairs
+│
+└── 🌐 Spaces
+    ├── playground      (interactive demo)
+    ├── model-kitchen  (model combination interface)
+    ├── dataset-explorer
+    └── community-hub