chore(release): add 0.4.2 changelog, sync bumpversion

The 0.4.1 → 0.4.2 version bump (a756295) was done manually and missed
.bumpversion.cfg; catching it up so the next bumpversion run picks up
the right base. Also writes the 0.4.2 release story into CHANGELOG.md
so the release workflow can extract it as the GitHub Release body.

.bumpversion.cfg

@@ -1,5 +1,5 @@
 [bumpversion]
-current_version = 0.4.1
+current_version = 0.4.2
 commit = True
 tag = True
 tag_name = v{new_version}