Switch Variant WDK elements from includeProjects to excludeProjects

The Short Variant record and its related WDK elements — the VariantRecordClasses recordClassSet, the <recordClass ref=...> entries in span_a/span_b, the variant searches, and anything else scoped the same way — currently list their projects with includeProjects.

Switch them to excludeProjects, and add a comment explaining why the excluded projects are excluded.

Why: with an include list, a newly added site is silently omitted until someone remembers to edit each of these lines. With an exclude list it inherits the feature by default, which is the right bias.

Watch out for: these have to stay consistent with each other. A <recordClass ref=...> may only name projects where VariantRecordClasses actually exists. If the refs become excludeProjects while the recordClassSet keeps an explicit include list, adding a new site leaves the refs pointing at a record class that does not exist there. Convert them together.

Open question for whoever picks this up: the current project list was inherited verbatim from SnpRecordClasses and no rationale was recorded, so the "why excluded" comment needs a real answer from someone who knows — not a plausible-sounding guess.

Raised by @steve-fischer-200 in review of #220.

Start by locating the Short Variant record, VariantRecordClasses recordClassSet, the entries in span_a/span_b, and the related variant searches. Before changing the project lists, ask someone familiar with the inherited SnpRecordClasses list why each project is excluded. Done means the elements use consistent excludeProjects settings, the refs only target projects where VariantRecordClasses exists, and comments explain the confirmed exclusions.