change: Implement encoder registry system

[mathysgrapotte]

Is your change request related to a problem? Please describe.

Encoders currently use hardcoded class names in YAML configs.

Describe the solution you'd like

1. Create src/stimulus/data/encoding/registry.py
2. Implement EncoderRegistry subclassing BaseRegistry
3. Add decorators to existing encoders:

 @encoder_registry.register("text_onehot")
 class TextOneHotEncoder: ...

4. Update YAML references from class names to registry keys