INSTALL.md: KF6 is packaged in MSYS2 UCRT64; use it as the nightly does

The Windows section said KDE Frameworks are not prebuilt for Windows by any
route and that KF6 is not packaged in MSYS2, so every Windows build was told
to use -DBUILD_WITH_KF=OFF. MSYS2 packages kcoreaddons and kwidgetsaddons for
its UCRT64 and CLANG64 environments (not MINGW64), and
.github/workflows/windows-build.yml builds with them (-DBUILD_WITH_KF=ON
-DBUILD_KF=OFF).

Switch the MSYS2 recipe from MINGW64 to UCRT64 with the workflow's packages and
options. This also adds qt6-svg, a required Qt component the old package list
left out. MINGW64 users are told to drop the KDE packages and build with
-DBUILD_WITH_KF=OFF, as before.

Reported in discussion #1565.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

INSTALL.md

@@ -227,9 +227,11 @@ for why this matters).
 `QtPdf` is available as an optional component in the Qt Online Installer
 (look for "Qt PDF" under the Qt 6.4+ node); if you skip it, PDF page import
 is silently disabled exactly as on the other platforms. KDE Frameworks are
-not prebuilt for Windows by any of the routes below, so Windows builds
-normally use `-DBUILD_WITH_KF=OFF` (see the
-[autosave caveat](#9-building-without-kde-frameworks)).
+prebuilt for Windows only in MSYS2's UCRT64 and CLANG64 environments (the
+UCRT64 route below, which is also what the official Windows build uses).
+With MSVC or any other MinGW, use `-DBUILD_WITH_KF=OFF` (see the
+[autosave caveat](#9-building-without-kde-frameworks)); otherwise the build
+downloads and compiles KDE Frameworks itself, which is slow.
 
 ### MSVC (Visual Studio 2019 / 2022)
 
@@ -259,25 +261,35 @@ Two common ways to get a MinGW toolchain that matches a Qt6 MinGW kit: Qt's
 own bundled MinGW (installed alongside a `mingw_64` Qt kit via the Qt
 Online Installer), or [MSYS2](https://www.msys2.org).
 
-Using MSYS2, from a **MINGW64** shell:
+Using MSYS2, from a **UCRT64** shell (the same packages and options as
+`.github/workflows/windows-build.yml`):
 ```sh
 pacman -S --needed \
-    mingw-w64-x86_64-toolchain \
-    mingw-w64-x86_64-qt6-base mingw-w64-x86_64-qt6-tools \
-    mingw-w64-x86_64-cmake mingw-w64-x86_64-ninja \
-    mingw-w64-x86_64-sqlite3
+    mingw-w64-ucrt-x86_64-toolchain \
+    mingw-w64-ucrt-x86_64-qt6-base mingw-w64-ucrt-x86_64-qt6-svg \
+    mingw-w64-ucrt-x86_64-qt6-tools mingw-w64-ucrt-x86_64-qt6-translations \
+    mingw-w64-ucrt-x86_64-cmake mingw-w64-ucrt-x86_64-ninja \
+    mingw-w64-ucrt-x86_64-pkg-config mingw-w64-ucrt-x86_64-sqlite3 \
+    mingw-w64-ucrt-x86_64-kcoreaddons mingw-w64-ucrt-x86_64-kwidgetsaddons \
+    mingw-w64-ucrt-x86_64-extra-cmake-modules
 
 mkdir build && cd build
-cmake .. -G Ninja -DBUILD_WITH_KF=OFF -DCMAKE_BUILD_TYPE=Release
+cmake .. -G Ninja -DCMAKE_BUILD_TYPE=Release -DCMAKE_PREFIX_PATH=/ucrt64 \
+    -DBUILD_WITH_KF=ON -DBUILD_KF=OFF \
+    -DCMAKE_POLICY_VERSION_MINIMUM=3.5 \
+    -DSQLite3_INCLUDE_DIR=/ucrt64/include \
+    -DSQLite3_LIBRARY=/ucrt64/lib/libsqlite3.dll.a
 cmake --build .
 ```
 
-If `libbacktrace` is installed in your MSYS2 environment, pass the following
+`-DBUILD_KF=OFF` makes the build use the KDE Frameworks packages just
+installed instead of downloading and compiling its own.
+
+If `libbacktrace` is installed in your MSYS2 environment, add the following
 cached CMake variables when configuring. They are only necessary in that
 case, because `FindBacktrace` needs them to locate the library:
 
 ```sh
-cmake .. -G Ninja -DBUILD_WITH_KF=OFF -DCMAKE_BUILD_TYPE=Release \
     -DBacktrace_INCLUDE_DIR=/c/msys64/clang64/include \
     -DBacktrace_LIBRARY=/c/msys64/clang64/lib/libbacktrace.a
 ```
@@ -287,7 +299,10 @@ and `Backtrace_LIBRARY` to the `libbacktrace.a` file. The paths above are the
 usual locations for the MSYS2 `clang64` environment; adjust them if your
 installation uses a different prefix.
 
-(KF6 isn't packaged in MSYS2 either, hence `-DBUILD_WITH_KF=OFF` again.)
+KF6 is packaged for MSYS2's UCRT64 and CLANG64 environments only. In a
+**MINGW64** shell (`mingw-w64-x86_64-*` packages) it is not available: install
+the same packages without the three KDE ones and configure with
+`-DBUILD_WITH_KF=OFF` instead of `-DBUILD_WITH_KF=ON -DBUILD_KF=OFF`.
 
 Using the Qt Online Installer's bundled MinGW kit instead: point
 `CMAKE_PREFIX_PATH` at that kit (e.g. `C:\Qt\6.x.x\mingw_64`) and make sure
@@ -332,7 +347,8 @@ platforms, including FreeBSD as of this writing.
 `BUILD_WITH_KF=OFF` builds QET against a bundled fallback UI layer instead
 of KDE Frameworks (`kcoreaddons`/`kwidgetsaddons`). This is a fully
 supported configuration and is, in practice, the default on platforms where
-KF6 isn't readily packaged (macOS and Windows in particular).
+KF6 isn't readily packaged (macOS, and Windows outside MSYS2's UCRT64 and
+CLANG64 environments).
 
 Almost everything works identically either way. The one known caveat:
 **autosave behavior is not as polished without KF**, since QET relies on