Implement Maven reproducible builds

[Copilot]

This PR implements Maven reproducible builds for the Jaxen project, ensuring that building the same source code multiple times produces bit-for-bit identical artifacts.

Changes Made

Core Implementation:

• Added project.build.outputTimestamp property set to 2024-01-15T00:00:00Z in the main pom.xml
• This property ensures all Maven plugins use a fixed timestamp for artifact generation

Verification & Documentation:

• Created verify-reproducible-build.sh script that builds the project twice and compares SHA256 hashes to verify reproducibility
• Updated README.md with a new "Reproducible Builds" section explaining the feature and how to verify it

Benefits

Reproducible builds provide several important advantages:

• Security: Enables verification that distributed binaries match the claimed source code
• Trust: Users can independently verify that artifacts haven't been tampered with
• Debugging: Identical builds make it easier to track down issues across different environments
• Compliance: Meets modern software supply chain security requirements

Testing

The implementation has been thoroughly tested:

• Multiple builds produce identical SHA256 hashes for all JAR artifacts
• Builds remain reproducible across different directories and environments
• All existing tests continue to pass
• The verification script successfully validates reproducibility

All Maven plugins used in the build are at versions that support reproducible builds (maven-compiler-plugin 3.13.0, maven-source-plugin 3.3.1, maven-bundle-plugin 5.1.9, etc.).

Fixes #173.

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