Sort MO indexes by encoded context and message keys

[fhgffy]

MO files require the original-string table to be sorted by its encoded lookup keys. Sorting Message objects orders msgid before context, which can emit an unsorted table when messages have contexts. Babel does not emit a hash table, so binary-search readers depend on this ordering.

Sort the existing descriptors by their serialized context/singular keys before writing the parallel original and translation indexes. Add a regression that checks the emitted tables and translation correspondence with UTF-8 and cp1252 contexts and plural messages.

Validation: the new regression fails on the original source; all 6 MO tests and the complete suite (7,829 passed, 10 skipped, 2 xfailed) pass after the fix. Project Ruff and whitespace/debug hooks and sdist/wheel builds passed.

Specification: https://www.gnu.org/software/gettext/manual/html_node/MO-Files.html