docs: 3.4.0 release notes

Built with `towncrier build --version 3.4.0`, consuming the 33 fragments
under changes/. Two touch-ups to the generated section: the codec
error-message example fence gets a `text` language (markdownlint MD040),
and the three fragments that had been named after issues rather than
the pull requests that merged them (3285 -> #4063; 4174 and 4272 ->
#4218) now link to those pull requests.

Assisted-by: ClaudeCode:claude-fable-5-1

Author: d-v-b