[Domain] Fix stair portal stall

[SilverSupplier]

Summary

• Fix stalled vertical stair transitions when an agent reaches just short of the portal plane after crowd pressure.
• Keep normal vertical crossing strict, and only apply the near-portal tolerance after the route has been stalled.
• Add regression coverage for a stalled vertical transition near the portal plane.

Related Issue

• Closes Task- U자 계단 연결부 끼임 개선 #180

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
• cmake --build --preset build-no-app-debug
• ctest --preset test-no-app-debug

Risks / Follow-up

• The tolerance is intentionally small and only applies after a stall, so agents still need to reach the stair portal before changing floors.