core: implement engine registry and selector

[heritechie]

Goal:
Enable multiple map engines to be registered and selected
without changing core editor code.

Tasks:

• Implement an engine registry keyed by engine name
• Allow engine selection via editor configuration
• Set Leaflet as the default engine
• Provide clear error handling for unknown engines

Acceptance:

• Editor can switch engines via config
• No code changes required to swap engines
• Invalid engine selection fails fast with a clear error message