Backfill CHANGELOG for 0.4 and 0.5

[theDakshJaitly]

Context

CHANGELOG.md entries stop at 0.3.5; 0.4 and 0.5 are missing. COMPATIBILITY.md points to the changelog as the surfacing mechanism for breaking changes — currently violated.

Scope

Backfill changelog sections for the 0.4.x and 0.5.0 releases, in the existing Keep-a-Changelog style and consistent with the current entries. Flag any compatibility-relevant changes explicitly.

Note — needs maintainer input

The contributor does the writing; the maintainer supplies the facts. Comment on this issue and @theDakshJaitly will provide the list of changes that shipped in 0.4 and 0.5.

Acceptance criteria

• CHANGELOG.md has 0.4 and 0.5.0 sections in the existing format.
• Compatibility-relevant changes are called out.