📝 Add changelog and upgrade guide fragments

Signed-off-by: burgholzer <burgholzer@me.com>

Author: burgholzer

CHANGELOG.md

@@ -25,6 +25,7 @@ This project adheres to [Semantic Versioning], with the exception that minor rel
 
 ### Changed
 
+- 📦 Build MLIR by default for C++ library builds ([#1356]) ([**@burgholzer**], [**@denialhaag**])
 - 📦🏁 Build Windows x86 wheels on windows-2025 runner for newer compiler ([#1415]) ([**@burgholzer**])
 - 👷 Build on `macos-15`/`windows-2025` by default and `macos-14`/`windows-2022` for extensive tests ([#1414]) ([**@burgholzer**])
 - 📦🍎 Build macOS arm64 wheels on macos-15 runner for newer compiler ([#1413]) ([**@burgholzer**])

UPGRADING.md

@@ -4,6 +4,20 @@ This document describes breaking changes and how to upgrade. For a complete list
 
 ## [Unreleased]
 
+### MLIR enabled by default for C++ builds
+
+The MLIR-based functionality within MQT Core has long been experimental and opt-in.
+Starting with this release, MLIR is enabled by default for C++ library builds.
+This means that LLVM (including MLIR) is now a required dependency for building MQT Core from source.
+
+We offer pre-built distributions for all supported platforms as part of the `setup-mlir` project at https://github.com/munich-quantum-software/setup-mlir.
+Please follow the instructions there to install the distribution for your platform.
+You can then point CMake to the installation directory using the `-DMLIR_DIR=/path/to/mlir/installation/lib/cmake/mlir` option.
+
+The MLIR components can still be manually disabled by passing `-DBUILD_MQT_CORE_MLIR=OFF` to CMake.
+MLIR is also not enabled for the Python package builds because no functionality depends on it yet.
+This is expected to change in the future.
+
 ### QDMI-Qiskit integration
 
 This release introduces a Qiskit `BackendV2`-compatible interface to QDMI devices.