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 entries whose fragments were named after issues rather
than pull requests (#3285, #4174, #4272) now link to /issues/ instead of
/pull/.

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

Author: d-v-b