engine: add secondary map engine (read-only)

[heritechie]

Goal:
Provide a second, real map engine implementation to validate
multi-engine support.

Scope (locked):

• Render map
• Render existing boundary (GeoJSON)
• No drawing or editing capabilities

Tasks:

• Choose a secondary map engine (e.g. Google Maps or MapLibre GL)
• Implement adapter conforming to MapEngineAdapter
• Expose capability flags reflecting read-only limitations
• Ensure engine can be selected via registry

Acceptance:

• Editor runs with at least two different map engines
• Boundary renders correctly on both engines
• Core logic remains engine-agnostic