PerryLink/dsh-hazchem-check

Checks a hazardous-chemical register for internal consistency: that each substance names its hazard class, that stored quantities parse, that a threshold is recorded, that the major-hazard-source verdict agrees with how the stored quantity compares to the threshold the register states, that safety data sheet numbers are present and CAS numbers are not duplicated. Reports differences for a human to review; it does not decide whether a unit constitutes a major hazard source.