[WIP] Add separate badges for Ubuntu, MacOS, and Windows workflows and aggregate coverage badge to README.md; update workflow for multi-OS testing

[Copilot]

• Explore repository structure and understand current CI setup
• Identify README.rst file (not README.md) with existing badges
• Examine current GitHub Actions workflow (Ubuntu only)
• Update GitHub Actions workflow to run on Ubuntu, MacOS, and Windows
• Configure coverage aggregation across OS platforms using Coveralls
• Update README.rst badges to show separate status for each OS platform
• Test workflow configuration and badge functionality
• Verify coverage reporting works correctly across all platforms

This pull request was created as a result of the following prompt from Copilot chat.

Update the README.md file to include separate GitHub Actions workflow badges for Ubuntu, MacOS, and Windows. Each badge should show the build status for its respective OS. Also, add a coverage badge from Coveralls that aggregates results from all OS runs if possible; if aggregation is not supported, default to showing coverage for Ubuntu only. Additionally, modify the GitHub Actions workflow to run tests on Ubuntu, MacOS, and Windows. Ensure that the workflows are configured to send coverage results to Coveralls and attempt to aggregate the coverage across all OS runs.

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