Add NuGet package caching to GitHub Actions workflows

[Copilot]

Addresses code review feedback to optimize CI build performance.

Changes

• NuGet Caching: Added actions/cache@v4 to both build jobs

  • Caches ~/.nuget/packages with keys derived from packages.lock.json and packages.config hashes
  • Includes fallback restore key for partial cache hits
  • Eliminates redundant package downloads on successive runs

• x64 Support: Verified existing matrix strategy already builds both Win32 and x64 platforms

- name: Cache NuGet packages
  uses: actions/cache@v4
  with:
    path: ~/.nuget/packages
    key: ${{ runner.os }}-nuget-${{ hashFiles('**/packages.lock.json', '**/packages.config') }}
    restore-keys: |
      ${{ runner.os }}-nuget-

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