feat(docs): remediate CAGE_ARXIV.md and enable safety-by-default (#79)

Audit: Verified 242 technical claims against codebase
- Fixed 10 discrepancies (line numbers, parameters, deprecated refs)
- Updated POAM resolutions (2026-038, 040, 041, 042)
- Addressed peer review concerns:
  - (A) Enable Redis WAIT by default (CAGE_REDIS_WAIT_REPLICAS=1)
  - (A) Enable fence epoch by default (CAGE_REDIS_SYNCHRONOUS_REPLICATION=true)
  - (B) Enable evidence chain blocking by default (EVIDENCE_CHAIN_BLOCKING=true)
  - (C) Add distributed model citation to proof/model.py
- Updated tmp/CAGE_ARXIV.md to reflect v3.1 safety defaults
- Moved paper artifacts from docs/paper/ and plans/ to tmp/

Author: lahlfors