[Domain] Fix single-exit recommendations

[muzygosu]

Summary

• 출구가 1개일때도 병목 없는 출구로 유도 대안이 등장하는 버그, 출구에 병목이 있으며 출구 사용량차이가 25% 이상일때 등장해야하는데 출구가 1개일때 100% VS 0%로 인식

• Prevent route-guidance recommendations that point to another exit when the scenario has fewer than two reachable exit candidates.

• Apply the same guard to exit balancing, pressure relief, and cross-flow route-guidance alternatives.

• Add regression coverage for single-exit bottlenecks and the blocked-relief ordering case under the new reachable-exit policy.

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 (full local Windows debug/app verification was not needed for this domain-only fix)

Additional checks run:

• GitHub Actions CI / Build and Test passed on PR commit 130e412.
• GitHub Actions PR Policy / Validate PR passed on PR commit 130e412.
• cmake --build --preset build-engine-domain-debug passed locally.
• git diff --check passed locally.

Risks / Follow-up

• This keeps the fix scoped to recommendation eligibility. It does not add or restore the temporary barrier/flow-deflector feature that was rolled back from the prior working branch.