ci: add Windows (MSYS2 UCRT64) CMake workflow building the full tree

CMake counterpart of the legacy qmake ci-ucrt64.yml, but builds the whole
tree (service/friendserver/sam3/rnp/plugins all ON) with Ninja and a shared
libretroshare, mirroring the user's validated local Windows command.
Submodules init without --remote (like the Linux/macOS CMake CIs).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/windows-cmake.yml

@@ -0,0 +1,107 @@
+# Windows (MSYS2 / UCRT64) CMake CI for the RetroShare monorepo.
+#
+# CMake counterpart of the legacy qmake ci-ucrt64.yml. Unlike that one (GUI-only,
+# sam3 disabled, mingw32-make), this builds the WHOLE tree with the same feature
+# set as the Linux/macOS CMake CIs, using Ninja and the user's validated local
+# Windows command (shared libretroshare).
+#
+# Submodules are initialised WITHOUT --remote (identical to ubuntu-cmake.yml) so
+# libretroshare stays on its pinned, CMake-enabled commit. On WIN32 libretroshare
+# pulls restbed via FetchContent, so the restbed submodule is unused there.
+#
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+name: Windows UCRT64 CMake C/C++ CI
+
+on:
+  push:
+    paths-ignore:
+    - '**/*.md'
+  pull_request:
+    paths-ignore:
+    - '**/*.md'
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    name: Windows UCRT64 / Qt5 / CMake
+    runs-on: windows-latest
+    defaults:
+      run:
+        shell: msys2 {0}
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          persist-credentials: false
+
+      - name: Setup MSYS2 (UCRT64)
+        uses: msys2/setup-msys2@40677d36a502eb2cf0fb808cc9dec31bf6152638 # v2.28.0
+        with:
+          msystem: UCRT64
+          update: true
+          install: >-
+            base-devel
+            git
+            doxygen
+            perl
+            ruby
+            mingw-w64-ucrt-x86_64-toolchain
+            mingw-w64-ucrt-x86_64-cmake
+            mingw-w64-ucrt-x86_64-ninja
+            mingw-w64-ucrt-x86_64-python
+            mingw-w64-ucrt-x86_64-qt5-base
+            mingw-w64-ucrt-x86_64-qt5-multimedia
+            mingw-w64-ucrt-x86_64-qt5-svg
+            mingw-w64-ucrt-x86_64-qt5-tools
+            mingw-w64-ucrt-x86_64-openssl
+            mingw-w64-ucrt-x86_64-libbotan
+            mingw-w64-ucrt-x86_64-sqlcipher
+            mingw-w64-ucrt-x86_64-xapian-core
+            mingw-w64-ucrt-x86_64-miniupnpc
+            mingw-w64-ucrt-x86_64-json-c
+            mingw-w64-ucrt-x86_64-rapidjson
+            mingw-w64-ucrt-x86_64-asio
+            mingw-w64-ucrt-x86_64-libxml2
+            mingw-w64-ucrt-x86_64-libxslt
+            mingw-w64-ucrt-x86_64-speex
+            mingw-w64-ucrt-x86_64-speexdsp
+            mingw-w64-ucrt-x86_64-ffmpeg
+            mingw-w64-ucrt-x86_64-bzip2
+            mingw-w64-ucrt-x86_64-zlib
+
+      # Same pinned-commit init as the Ubuntu/macOS CMake CI (no --remote); librnp
+      # needs --recursive for its nested src/libsexpp.
+      - name: Init submodules (pinned commits)
+        run: |
+          git submodule update --init \
+            libbitdht/ libretroshare/ openpgpsdk/ retroshare-webui/
+          git submodule update --init --recursive \
+            supportlibs/librnp
+          git submodule update --init \
+            supportlibs/restbed supportlibs/libsam3 \
+            supportlibs/udp-discovery-cpp supportlibs/rapidjson
+          git --no-pager -C libretroshare log --max-count 1 --oneline
+
+      # Mirrors the validated local Windows command (Ninja, shared libretroshare).
+      - name: CMake configure
+        run: >
+          cmake -G Ninja -B Build-cmake -S .
+          -DCMAKE_POLICY_VERSION_MINIMUM=3.5
+          -DCMAKE_BUILD_TYPE=Release
+          -DRS_LIBRETROSHARE_STATIC=OFF -DRS_LIBRETROSHARE_SHARED=ON
+          -DRS_RNPLIB=ON -DRS_JSON_API=ON -DRS_WEBUI=ON
+          -DRS_GUI=ON -DRS_SERVICE=ON -DRS_FRIENDSERVER=ON
+          -DRS_PLUGINS=ON -DRS_FORUM_DEEP_INDEX=ON
+          -DRS_USE_I2P_SAM3=ON -DRS_BITDHT=ON -DRS_MINIUPNPC=ON
+          -DRS_BRODCAST_DISCOVERY=ON -DRS_SQLCIPHER=ON
+
+      - name: CMake build
+        run: cmake --build Build-cmake -j3