fix(xlsform): a guidance_hint pyxform rejects in parameters becomes its column

[jstet]

qwacback's old DDI → XLSForm export stored the guidance hint inside parameters as guidance_hint=<text>. pyxform reads parameters as space-separated key=value pairs, so a hint with spaces makes the form invalid. Since #163, ddi2xlsform gives parameters back exactly as authored, including that invalid cell, and Kobo would refuse the result.

Now the XLSForm writer moves such a hint, one whose text has whitespace, to the guidance_hint column. A hint without whitespace stays in parameters, so a valid form still comes back as written. The codebook doesn't change: cdl:parameters stays as authored.

• hints_survey's ddi2xlsform.json is valid XLSForm again, so its pyxform xfail is removed. Its source stays an expected failure.
• Round-trip tests: the canonical comparison folds this repair. For a repaired form, the DDI → XLSForm → DDI codebook changes once and is then stable.
• New unit tests cover both cases: a hint that stays in parameters, and one that moves.
• ddi2xlsform/README.md lists it under known losses as a repair.

Checks

• vitest: 1475 passed
• pytest validation + codegen: 151 passed, 3 xfailed
• npm run validate and the drift check pass

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