refactor(core): extract boundary state manager

[heritechie]

Goal:
Separate boundary data and validation state from rendering and engine logic.

Tasks:

• Introduce a boundary state manager / store
• Manage:
  • current boundary (GeoJSON)
  • empty / dirty state
  • validation result
• Remove direct boundary mutation from engine adapter

Acceptance:

• Boundary logic is testable without a map engine
• Engine adapters only emit boundary change events
• Core editor owns boundary state lifecycle