Function build_block_to_agent_map 

pub(crate) fn build_block_to_agent_map(
    discovery: &Value,
) -> HashMap<String, String>

FleetBroadcast’s block_id -> agent-name resolution (REPORT_CROSS_INSTANCE_CONTROL_ROBUSTNESS_AUDIT_2026_08_22.md): a /agentmux/discovery response’s host.addressable AND host.cross_channel sections both carry a block_id — addressable under agent_id/block_id, cross_channel (a different channel on this same host) under name/block_id. A block_id is unique across channels on one host, so both are folded into one map, not kept separate. lan/wan entries carry no block_id at all (they’re not local blocks) — a target for those tiers is never in this map, and the caller falls back to treating it as a literal agent name instead (see the FleetBroadcast handler).

Pure (no I/O) — extracted so it’s unit-testable without a live discovery endpoint.