Add modern .elpx parsing and content inspection APIs

[erseco]

Summary

This PR expands the parser from legacy .elp support into the modern ODE-based project family and documents the new behavior.

Changes

• add support for modern content.xml packages in both .elp and .elpx
• keep legacy contentv3.xml support for eXeLearning 2.x projects
• add format inspection APIs for extension, content file, content format, schema version, resource layout, root DTD and likely v4 detection
• parse modern pages, blocks, idevices, grouped page texts and visibility flags
• add resource inspection APIs for images, audio, video, documents, detailed asset origins and orphan assets
• harden archive extraction to avoid unsafe ZIP paths
• add two real .elpx fixtures covering modern v4-style packages
• update README and docs to describe .elp / .elpx, modern ODE packages and the new APIs

Notes

• modern .elpx packages with root content.dtd are currently treated as likely v4-style packages
• exact major-version detection is still limited by what the package metadata exposes internally

Validation

• composer test
• composer lint