Display data (alarm_value, current_value, limit_up/down, items, unit) now
goes to a local-memory DisplayCache and is serialized to JSON without any
shared memory lock. Cold data (stat_values, running_time, shear_times, etc.)
stays in shared memory for mon-cron IPC, protected by a real interprocess
mutex (boost::interprocess::interprocess_mutex) instead of the broken
process-local std::mutex. AlgBase::rule_stat_ is now RuleStatLocal with
standard types — zero changes to algorithm subclass code.
973921fc4b