feat: add phase 4 stability, collector, and rerank evidence

[Chloride233]

Summary

• add Phase 4 stability, cost, and user-study tooling with reproducible evaluation artifacts
• add the opt-in Go collector path while preserving Python as the default and fallback
• add optional reranking, blind review tooling, reproducible evaluation, and recall-coverage audit assets
• align README, contributing guidance, Agent instructions, crawler tiers, API route counts, and frontend view counts with the repository

Verification

• PYTHONPATH=src .venv/bin/python -m pytest -q tests/unit/test_crawler_audit.py tests/unit/test_reranker.py tests/unit/test_recall_coverage_audit.py -p no:cacheprovider — 20 passed
• git diff --check — passed
• one existing Pydantic class-config deprecation warning remains outside this PR's cleanup scope

Risk / rollback

The Go collector and reranking paths are opt-in; default Python collection remains available. Evaluation artifacts are intentionally large and evidence-oriented. Production stability and real-user value remain pending and are not claimed as complete.

Docs / status

The crawler support boundary remains 3 Full Support plus 6 Experimental platforms. README and governance files now point to the current counts and authoritative crawler audit.