Adds Vcpkg setup to CI workflows

Adds vcpkg setup for Arch Linux and Windows CI jobs.

This ensures consistent dependency management and builds via Vcpkg for both platforms and enables OMath to build via vcpkg.
Clones the vcpkg repository and bootstraps it during the job execution.

.github/workflows/cmake-multi-platform.yml

@@ -19,6 +19,9 @@ jobs:
     name: Arch Linux (Clang)
     runs-on: ubuntu-latest
     container: archlinux:latest
+    env:
+      VCPKG_ROOT: ${{ github.workspace }}/vcpkg
+      OMATH_BUILD_VIA_VCPKG: ON
 
     steps:
       - name: Install basic tool-chain with pacman
@@ -33,6 +36,12 @@ jobs:
         with:
           submodules: recursive
 
+      - name: Set up vcpkg
+        shell: bash
+        run: |
+          git clone --depth 1 https://github.com/microsoft/vcpkg "$VCPKG_ROOT"
+          "$VCPKG_ROOT/bootstrap-vcpkg.sh" -disableMetrics
+
       - name: Configure (cmake --preset)
         shell: bash
         run: cmake --preset linux-release -DOMATH_BUILD_TESTS=ON -DOMATH_BUILD_BENCHMARK=OFF
@@ -47,12 +56,15 @@ jobs:
 
 
 
-##############################################################################
-# 2)  Windows  –  MSVC / Ninja
-##############################################################################
+  ##############################################################################
+  # 2)  Windows  –  MSVC / Ninja
+  ##############################################################################
   windows-build-and-test:
     name: Windows (MSVC)
     runs-on: windows-latest
+    env:
+      VCPKG_ROOT: ${{ github.workspace }}\vcpkg
+      OMATH_BUILD_VIA_VCPKG: ON
 
     steps:
       - name: Checkout repository (with sub-modules)
@@ -66,6 +78,12 @@ jobs:
       - name: Set up MSVC developer command-prompt
         uses: ilammy/msvc-dev-cmd@v1
 
+      - name: Set up vcpkg
+        shell: pwsh
+        run: |
+          git clone --depth 1 https://github.com/microsoft/vcpkg $env:VCPKG_ROOT
+          & "$env:VCPKG_ROOT\bootstrap-vcpkg.bat" -disableMetrics
+
       - name: Configure (cmake --preset)
         shell: bash
         run: cmake --preset windows-release -DOMATH_BUILD_TESTS=ON -DOMATH_BUILD_BENCHMARK=OFF