Move the Unreleased entries into 0.2.40, dated 2026-09-20, add the
full-diff link, set KPlugin.Version to the released version, and align
the README capture caption.

Documentation impact: CHANGELOG.md moves already-reviewed Unreleased
entries (verified against the v0.2.39..HEAD scope) into the versioned
section; README caption follows the metadata bump. No guide or TODO
changes: the release ships the behavior those guides already describe.