Add readOnly parameter to preload() for readonly queries

[JanTvrdik]

Summary

• Adds a new optional readOnly parameter to the preload() method
• When readOnly: true, applies Query::HINT_READ_ONLY to inner queries
• Explicitly marks preloaded entities as read-only in the UnitOfWork
• Improves performance for read-only operations by disabling change tracking

Usage

// Normal preload (entities are tracked for changes)
$preloader->preload($articles, 'category');

// Read-only preload (entities are not tracked for changes)
$preloader->preload($articles, 'category', readOnly: true);

Closes #36

Test plan

• Added tests for many-to-one associations (testManyHasOneWithPreloadReadOnly)
• Added tests for one-to-many associations (testOneHasManyWithPreloadReadOnly)
• Added tests for many-to-many associations (testManyHasManyWithPreloadReadOnly)
• All 252 tests pass
• PHPStan reports no errors

[janedbal]

Missing readme mention.