ReadAsync walked the whole batch sequentially, so one SLOW CNC (TCP-accept-then-stall,
the live 31i-B case) cost Timeout per tag per poll while no other device was read.
ReadAsync now resolves every reference against one table snapshot, buckets the rest by
device and reads the devices with Task.WhenAll; each device's tags still go in order on
its own serialised socket (the PMC coalescing pre-pass runs per device, first). Results
land at their own indices.
Also precomputes each device's fixed-tree key prefix (it was built per reference, twice
per poll).
Refs #740
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Joseph Doherty
256915b7e4