[relay] Trust layer — multi-validator quorum and XERIS-side output_hash verification

[Dipraise1]

Context

Relay payloads are currently signed by a single validator hotkey (engram/relay/signer.py). A single compromised validator can inject arbitrary 'intelligence' into XERIS.

Tasks

• Multi-validator quorum — XERIS accepts a payload only when at least 2 validators have submitted matching output_hash signatures for the same (netuid, block, output)
• XERIS-side verification — verify output_hash against Engram on-chain state before accepting

Notes

• Payloads already carry output_hash, netuid, block, and a replay nonce, so the aggregation key exists
• Quorum aggregation could live XERIS-side (collect N signatures) or Engram-side (aggregate before submit) — decide and document

From TASKS.md — Relay Phase 3, open items.