PoE record fields

{
  "agent":      "0x8f3a…c21e",          // agent identity (hash)
  "from":       14, "to": 15,            // versions
  "eval_hash":  "sha256:9b1f…",          // eval-set hash; content never goes on-chain
  "holdout":    { "pass": true, "n": 120 },
  "score":      { "before": 0.61, "after": 0.74, "sigma": 0.02 },
  "layers":     ["context", "skills"],   // layers changed this round
  "nodes":      { "evaluated": 5, "agree": 5 },
  "anchor":     "sg-01",                 // arbitrating anchor (if any)
  "compute":    { "gpu_hours": 3.2, "cost_usd": 4.10 },
  "sig":        ["0x…", "0x…"],
  "ts":         1791600000
}