🔖 Prepare release of v3.4.0 (#823)

## Description

This PR prepares the release of `v3.4.0`.

## Checklist:

- [x] The pull request only contains commits that are focused and
relevant to this change.
- [x] ~I have added appropriate tests that cover the new/changed
functionality.~
- [x] ~I have updated the documentation to reflect these changes.~
- [x] ~I have added entries to the changelog for any noteworthy
additions, changes, fixes, or removals.~
- [x] ~I have added migration instructions to the upgrade guide (if
needed).~
- [x] The changes follow the project's style guidelines and introduce no
new warnings.
- [x] The changes are fully tested and pass the CI checks.
- [x] I have reviewed my own code changes.

Author: denialhaag

CHANGELOG.md

@@ -9,6 +9,10 @@ This project adheres to [Semantic Versioning], with the exception that minor rel
 
 ## [Unreleased]
 
+## [3.4.0] - 2026-01-13
+
+_If you are upgrading: please see [`UPGRADING.md`](UPGRADING.md#340)._
+
 ### Changed
 
 - ♻️ Migrate Python bindings from `pybind11` to `nanobind` ([#817]) ([**@denialhaag**])
@@ -106,7 +110,8 @@ _📚 Refer to the [GitHub Release Notes] for previous changelogs._
 
 <!-- Version links -->
 
-[unreleased]: https://github.com/munich-quantum-toolkit/qcec/compare/v3.3.0...HEAD
+[unreleased]: https://github.com/munich-quantum-toolkit/qcec/compare/v3.4.0...HEAD
+[3.4.0]: https://github.com/munich-quantum-toolkit/qcec/releases/tag/v3.4.0
 [3.3.0]: https://github.com/munich-quantum-toolkit/qcec/releases/tag/v3.3.0
 [3.2.0]: https://github.com/munich-quantum-toolkit/qcec/releases/tag/v3.2.0
 [3.1.0]: https://github.com/munich-quantum-toolkit/qcec/releases/tag/v3.1.0

UPGRADING.md

@@ -4,6 +4,8 @@ This document describes breaking changes and how to upgrade. For a complete list
 
 ## [Unreleased]
 
+## [3.4.0]
+
 ### Python wheels
 
 This release contains two changes to the distributed wheels.
@@ -83,7 +85,8 @@ Alternatively, CMake can be conveniently installed from PyPI using the [`cmake`]
 
 <!-- Version links -->
 
-[unreleased]: https://github.com/munich-quantum-toolkit/qcec/compare/v3.3.0...HEAD
+[unreleased]: https://github.com/munich-quantum-toolkit/qcec/compare/v3.4.0...HEAD
+[3.4.0]: https://github.com/munich-quantum-toolkit/qcec/compare/v3.3.0...v3.4.0
 [3.3.0]: https://github.com/munich-quantum-toolkit/qcec/compare/v3.2.0...v3.3.0
 [3.2.0]: https://github.com/munich-quantum-toolkit/qcec/compare/v3.1.0...v3.2.0
 [3.1.0]: https://github.com/munich-quantum-toolkit/qcec/compare/v3.0.0...v3.1.0