Clarify how whitespace should be treated in secondary instance property names and values

This currently varies by implementation. Notably, Enketo strips whitespace from CSV column header names.

I think we should preserve values exactly and reject files with leading/trailing whitespace in property names.

The spec phrasing would be:
* for CSV: Column names MUST NOT contain leading or trailing whitespace.
* for geojson: Members of `properties` MUST NOT contain leading or trailing whitespace in their names.
* values: Processors MUST preserve whitespace in values exactly as represented in the source document.

Locate the specification sections covering CSV column names, GeoJSON properties, and secondary instance values, then review their current whitespace wording. Done means the text consistently rejects leading or trailing whitespace in property names and preserves value whitespace exactly for both formats.

Written by the indexing model from the issue text.