How to filter on taxonomy term but only match one field on the node, where two fields (primary and secondary) both use the same vocabulary
Ok - This is also out of the box Taxonomy Term behavior.  My issue was I was selecting the wrong option for what I wanted.  I wanted the behavior that converts term string to tid, and so the full solution is:


Contextual filter on Content: Primary Category field
Specify Validation > Taxonomy Term, my vocabulary
Select the option Term name converted to Term ID


Bam!  Success!  Now I get my list of nodes matching that vocabulary's terms as strings but only for the one of the multiple taxonomy fields of that vocabulary on the node.  To further exemplify, my next step will be to create another View that is all the Secondary matching terms as "related" but not primary like this one is.