Backfill changelog from GitHub release notes

Import all 53 published releases (v1.0.15 through v3.4.2) from the
auto-generated GitHub release notes: one dated entry per version,
pre-releases labeled, body headings demoted one level so version
headings stay the top-level structure.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: lelia <2418071+lelia@users.noreply.github.com>

Author: lelia