Add Windows/Mac platforms to CI test matrix with Java 8/11

[Copilot]

This PR expands the CI test matrix to include more platforms while maintaining Java compatibility as requested in the issue.

Changes Made

Expanded CI Matrix:

• Java versions: Java 8 and 11 (avoiding Java 17+ compatibility issues with source/target 1.6)
• Platforms: Added Windows and macOS (previously only Ubuntu)
• Total configurations: Expanded from 2 to 6 test combinations

CI Updates:

• Updated CI artifact naming to include OS for better organization

Matrix Overview

The new test matrix covers:

• Java 8, 11 on Ubuntu, Windows, macOS
• This ensures XOM works correctly across supported Java versions and major operating systems
• Maintains Java 1.6 source/target compatibility as required

Compatibility Note

Java 17+ no longer supports source/target 1.6 (minimum is 1.7), so the CI matrix focuses on Java 8/11 to avoid compatibility conflicts while still expanding platform coverage significantly.

Fixes #221.

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