fix: clarify rewrite debug score contract

[eunaverse]

Summary

• clarify rewrite gating and debug score semantics around initial vector scoring
• compare original vs rewritten result sets once and keep the better set
• align public debug opt-in contract across search, answer, docs, and tests

Validation

• uv run pytest -q tests/search/test_context_service.py -k rewrite
• uv run pytest -q tests/search/test_answer_service.py -k rewrite
• uv run pytest -q tests/api/test_tools_contract.py -k "debug or answer_with_citations_can_include_debug_payload"
• uv run pytest -q tests/api/test_tools_contract.py -k "structured_debug_payload or return_contract_shapes or legacy_removed_source_rows"
• uv run mypy search/retrieval_pipeline.py
• python -m compileall api core environments fetching indexing search storage main.py
• ./scripts/verify_functional_e2e.sh

Notes

• related issue 없음