chore: restructure the txt parse loop and reword the remaining echoes from the twelfth audit - #1897

Summary

Follow-up to the twelfth independent audit of the #1835 removal claims: it caught the third by details docstring the two earlier fixes missed, a TXT parse loop with five statements byte-identical to the 2009 setText, and a comment paraphrasing a 2009 changelog line.

Details

• async_all_by_details was the third sibling of the docstring family chore: drop the equality docstring family and reword the cache lookup docstring #1892 and chore: reword the remaining by-details docstring flagged by the eleventh audit #1896 addressed; its first line is reworded the same way, with the thread-safety paragraph untouched.
• _unpack_text_into_properties walked its length-prefixed bytes with the 2009 statement sequence (end/index, two separate increments). The loop now slices from a computed start and advances pos in one step; the operation count is unchanged (three additions per entry either way) and the .pxd locals are updated in the same commit. The test_txt_properties CodSpeed benchmark covers this path and will referee it on the PR.
• The query handler's multicast comment shared its wording shape with a line from the removed 2009 header changelog, and its old "75% of ttl" parenthetical described a check add_mcast_question_response does not perform. The comment now states what the code does: the answer also goes out by multicast for a unicast source, and a copy multicast within the last second is folded into the aggregated response per RFC 6762 section 14.

A four-agent simplify review ran on this diff: reuse, simplification and efficiency came back clean (the loop is op-count equivalent and the .pxd typing follows the repo's unsigned convention); the altitude reviewer caught the inaccurate first version of the comment reword above, which is fixed here.

Test plan

• full compiled test suite after REQUIRE_CYTHON=1 rebuild: 518 passed
• txt properties benchmark passes locally; CodSpeed will confirm no regression
• pre-commit clean (cython-lint included, since the .pxd changed)