[codex] Add scalable SAT snapshot miner path

[wallscaler]

Summary

• add /sat/latest.json signed pointer for the SAT miner read path
• add current-sequence SAT board and weights snapshots under /sat/sequences/{sequence}/...
• add /sat/events SSE hints for sequence/latest-url broadcast with heartbeats
• add /v1/agents/receipts/{receipt_id} for durable submit status lookup
• document this as the current compatibility slice before object storage/outbox work

Compatibility

• does not change /v1/validator/weights/next
• does not change SAT scoring semantics
• does not change existing submit payloads or miner auth
• keeps legacy read endpoints in place
• split service roles now allow the new read snapshot routes and submit receipt route

Codex Review Loop

• found and fixed a split-role blocker: /sat/* would have been blocked on read services and receipt lookup would have been blocked on submit services
• rebased cleanly onto current origin/main
• checked that sequence identity is bound to both board and weights hashes, not only policy version
• checked that SSE is a real stream with keepalives, with once=true only for bounded tests
• checked that submit receipt lookup does not expose raw signatures or solution bodies
• remaining limitation: historical immutable object storage, CNF artifact publishing, transactional outbox, and async verification workers are still follow-up phases

Tests

• wsl -e bash -lc 'cd /mnt/c/Users/fred/code/cathedral-pr289-review && /mnt/c/Users/fred/code/cathedral/.venv/bin/python publisher_verify.py' PASS, 157 checks
• wsl -e bash -lc 'cd /mnt/c/Users/fred/code/cathedral-pr289-review && /mnt/c/Users/fred/code/cathedral/.venv/bin/python assigned_lane_verify.py' PASS, 15 checks
• wsl -e bash -lc 'cd /mnt/c/Users/fred/code/cathedral-pr289-review && /mnt/c/Users/fred/code/cathedral/.venv/bin/python weights_verify.py' PASS, 32 checks
• wsl -e bash -lc 'cd /mnt/c/Users/fred/code/cathedral-pr289-review && /mnt/c/Users/fred/code/cathedral/.venv/bin/python -m compileall -q scaffold/publisher publisher_verify.py assigned_lane_verify.py weights_verify.py' PASS
• git diff --check PASS