[vagenas]

Three places in doclang migrate (and version handling) used Python's notion of whitespace or of the end of a string, not XML's. The spec defines whitespace as space, tab, CR and LF only.

• 0.8 to 0.9: a <content> ending in a line break was unwrapped, because ^…$ also matches before a trailing \n. The line break was lost:

  <text>one<content> two
  </content>three</text>

  migrated to <text>one two\nthree</text>, which 0.9 reads as one two three. It now keeps the <content>.
• Down steps (0.9 to 0.8, 0.8 to 0.7): removing a dropped element used a bare .strip(), which also strips a non-breaking space. A   between a dropped element and whitespace was lost. Only XML whitespace is stripped now.
• Version and name checks (_schemas, _api, _plan, _archive) use fullmatch instead of ^…$. E.g. version="0.8
" is now rejected as an invalid declared version, rather than failing later with a confusing "No schema for the document's version 0.8⏎".

Regression tests for each case; full test suite, ruff and mypy pass.