Function agent_zone_for_block_meta 

pub fn agent_zone_for_block_meta(
    meta: &HashMap<String, Value>,
) -> Option<String>

Resolve the agent’s GLOBAL agent:<defId>:current zone from a block’s meta.

The block’s agentId meta IS the agent definition_id (the same value the snapshot RPCs and blockfile:read_range fallback key on — see app_api.rs), so the zone the hot-path mirror writes and the zone the read fallback reads are identical by construction. Returns None when the block isn’t agent-anchored or carries an invalid id (no mirror/fallback).