buildDdiXml silently drops hint and guidance_hint

[jstet]

buildDdiXml drops the XLSForm hint column and parameters: guidance_hint=… without any warning, and validateSubset doesn't flag them either. Checked with v0.1.7:

buildDdiXml(
  [{ type: 'integer', name: 'a', label: 'A', hint: 'In years', parameters: 'guidance_hint=ask twice' }],
  [], { settings: {} },
);
// <qstn responseDomainType="numeric"><qstnLit>A</qstnLit></qstn>  — no preQTxt, no ivuInstr

Impact downstream. qwacback switched its XLSForm → DDI path to formtransform (CorrelAid/qwacback#3). Its old Go converter mapped:

• hint → <qstn><preQTxt>
• guidance_hint → <qstn><ivuInstr>

Both are now lost (CorrelAid/qwacback#12). qwacback's DDI → XLSForm direction still maps preQTxt → hint and ivuInstr → guidance_hint, so the round trip no longer holds. A note's text, which formtransform folds into the next question's preQTxt, also comes back as that question's hint.

Ask. One of:

1. Support them: hint → preQTxt (merged with any folded note text, if both are present) and guidance_hint → ivuInstr, registered like the other columns. Or:
2. If they're deliberately out of scope, have validateSubset report them (a warning at least) instead of dropping them silently, so that callers like qwacback's ddi-emitter can tell users.

qwacback's equivalence test already has a hint_and_guidance case (CorrelAid/qwacback#16) and will pick up the change once the pin is bumped.