docs: document multi-engine architecture

[heritechie]

Goal:
Provide minimal internal documentation explaining the
multi-engine architecture introduced in v0.4.0.

Tasks:

• Add a short README section covering:
  • Core vs engine responsibility
  • MapEngineAdapter role
  • Engine capability model
• Keep documentation concise and implementation-focused

Acceptance:

• Architecture can be understood in under 5 minutes
• Documentation explains "why", not just "what"
• No tutorials or framework-specific examples included