Transaction coordinator for MULTI/EXEC.
Raft-enabled transactions submit a single Raft log entry to an "anchor shard"
containing commands for all involved shards. The StateMachine's apply/3
writes to all shards' ETS tables and Bitcask files in one deterministic pass.
This includes single-shard write transactions; otherwise they would bypass
the quorum write path and only mutate the local shard process.
WATCH conflict detection
WATCH uses per-key tokens rather than per-shard write-version counters. Hot keys include the in-memory value hash plus the live Bitcask location; cold keys snapshot the live keydir location, so large values do not have to be materialized just to enter or check WATCH.