feat(ddi): every group a varGrp, variables in survey order (#152)

[jstet]

Closes #152. Part 2 of #155.

What changes

• Every group is a <varGrp>.
  • A plain group uses the XSD's own type="section", as agreed with qwacback; type="other" stays reserved for the semi-open pair.
  • A section lists its direct <var>s in @var and its direct child groups in @varGrp. Child groups are sections, grids, a select_multiple's multipleResp, and an other pair's varGrp.
  • The label goes in txt (one per xml:lang) and in concept.
  • @ID is VG_<path> and @name is the group path, as grids already did.
  • A group with no variable under it (e.g. only notes) gets no varGrp.
• Group conditions (follow-up to Convention: carry relevant/constraint in DDI as typed <notes> plus <universe> prose #151):
  • A group's own relevant is now the section's (or grid's) <universe> plus a cdl:relevant note.
  • Each cdl:relevant note carries only its element's own condition. That's what the way back (DDI round trips: DDI → Instrument parser, ddi2xlsform, and a round-trip contract #154) needs.
  • A variable's <universe> still states its groups' conditions too, so a reader sees who was asked.
• Survey order: <var>s follow the survey instead of being bucketed as grids, then multi-selects, then pairs, then the rest. The data CSV columns (getDdiColumnNames) follow the same order, so header = <var> order still holds.

<varGrp ID="VG_person" name="person" type="section" var="V_beruf V_bildung" varGrp="VG_sprachen VG_quelle">
  <txt>Person</txt>
  <txt xml:lang="en">Person</txt>
  <concept>Person</concept>
</varGrp>

Behaviour to be aware of

• Column order changes. The data CSV's column order changes for any form that isn't already in role order. Names and API are unchanged, and the header still matches <var> order.
• Sections are new elements. qwacback skips section today (qwacback#37).
• LimeSurvey's "Questions" wrapper: LimeSurvey needs every question in a group, so xlsform2lstsv wraps top-level questions in one called "Questions". lstsv2ddi can't tell it from an authored group, so it becomes a section. lstsv2ddiRoundtrip.test.ts ignores that one wrapper.

Still open (#153)

• The group hint.
• Keeping a note row separate from the hint that follows it: this comes with the postQTxt hint move.

Checks

• vitest: 1243 passed. New: tests/ts/unit/ddi/structure.test.ts, covering nesting, grids inside sections, empty groups, languages, and var and column order.
• pytest validation and codegen: 127 passed, 3 xfailed.
  • The XSD and Schematron checks pass on every snapshot.
  • The two KNOWN_INVALID fixtures fail with the same errors as before.
  • The Schematron varGrp/@type list now includes section, taken from the registry (type:begin_group).
• npm run validate and the drift check are clean.

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