engine(leaflet): move Leaflet logic into adapter

[heritechie]

Goal:
Isolate all Leaflet-specific logic inside a LeafletEngineAdapter
that implements MapEngineAdapter.

Tasks:

• Implement LeafletEngineAdapter
• Move Leaflet map initialization & teardown into adapter
• Move draw/edit controls and event bindings into adapter
• Emit boundary change events to core via adapter callbacks

Acceptance:

• Existing demo behavior remains identical
• No Leaflet imports exist in core modules
• Leaflet adapter fully supports drawing and editing polygons