按根本原因分组的 CommonMark 规范差距,以及有关查找表的问题

I ran the spec suite on `main` (18.0.9, 681373c) and went through the cases carrying `shouldFail` to see what is actually left. Since `update-specs.js` sets that flag from whatever fails at the time it runs, the set is a snapshot of current behavior rather than a list of decisions, so I grouped it by root cause. Posting it in case it is useful for triage, and because two of the groups run into the same wall and that seems worth asking about before anyone starts on them.