Add 4.15.3 changelog

Editor assessment

The subject is complete for its kind — the 4.15.3 changelog is added — though the body's first bullet contradicts it (`docs: add 4.15.13 changelog`, an apparent typo in the version), and the rest is a squash-merge bullet dump of repeated changelog-file updates with co-author trailers and content-free `update` lines. Type `docs` is correct; scope is unnecessary. Footers are well formed where present. The message is clear and honest at the category level, but the noisy body and the internal version inconsistency hold the teaching value at mediocre.