[Engine] tighten EngineWorld query boundary

[learncold]

Summary

• tighten the WorldQuery access boundary so upper layers obtain query access through EngineWorld rather than constructing ECS-backed handles directly
• move EngineWorld into a dedicated header and restrict its construction to engine-internal paths used by EngineRuntime and internal test helpers
• update engine tests, architecture docs, UML, and submission docs to match the final facade boundary

Related Issue

• Closes Task-EngineWorld 경유 WorldQuery 접근 경계 정리 #70

Area

• Engine
• Domain
• Application
• Docs
• Build
• Analysis
• Chore

Architecture Check

• I kept the dependency direction application -> domain -> engine.
• I did not add Qt UI code to src/domain.
• I did not add domain or application dependencies to src/engine.
• I used src/ as the include root.

Verification

• cmake --preset windows-debug
• cmake --build --preset build-debug
• ctest --preset test-debug
• Not run (reason below)

Risks / Follow-up

• none identified beyond normal CI confirmation on the PR