Add upstream compatibility and robustness regression corpus

[erseco]

Summary

• add a deterministic robustness corpus for malformed XML, encoded/Unicode asset references, traversal-like paths, malformed iDevice state and cyclic page hierarchies
• add an upstream fixture corpus runner that discovers real .elp / .elpx projects containing content.xml or contentv3.xml
• compare lightweight inspection with full parsing for upstream fixtures
• add a scheduled weekly Upstream Compatibility workflow
• use sparse checkout to download only upstream test/fixtures
• run the upstream workflow in this PR when its workflow/runner changes, while avoiding the cost on unrelated future PRs
• document local and scheduled compatibility testing

Stack

This PR is stacked on #19 and targets feature/typed-project-api.

Regular CI

The deterministic regression corpus passes on PHP 8.0–8.5, coverage, Composer validation, PHPCS and PHPStan.

Upstream CI

Opening this PR triggers the dedicated Upstream Compatibility workflow against current fixtures from exelearning/exelearning.