[relay] Finality guard — only relay after 3 block confirmations

[Dipraise1]

Context

The Engram → XERIS relay (engram/relay/) currently emits payloads from neurons/validator.py immediately after set_weights(). If the Engram chain reorgs, XERIS may consume outputs that never finalized.

Task

Add a finality guard: hold payloads in a pending queue and only submit to XERIS after the originating block has 3 confirmations on the Engram side.

Acceptance criteria

• Payloads queued with their origin block number; released only when current_block >= origin_block + 3
• Queue persists across validator restarts (reuse the existing SQLite relay log)
• Reorged-out payloads are dropped and logged, not submitted
• Tests covering confirmation release and reorg drop

From TASKS.md — Relay Phase 2, last open item.