[3.x] Support replacing discovered page classes

[emmadesilva]

Summary

• Add an early-bound API for replacing discovered page classes with application subclasses
• Resolve replacements only when assembling the discovery registry and constructing parsed pages
• Make file collection page-class queries polymorphic, matching page collection behavior
• Preserve subclass-aware Blade parsing, routing, sitemap classification, and heading permalinks
• Document behavioral replacement constraints and the exact-file-query upgrade case

Testing

• FeatureFramework: 1,898 passed; one sandbox-only socket binding failure
• UnitFramework: 1,934 passed; one known PHP 8.5 FeaturedImage failure
• FeatureHyde: 6 passed
• Focused replacement, extension, file collection, parser, route, sitemap, and permalink tests pass
• HydeStan reports no errors

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (cac9b84) to head (a1edd37).
⚠️ Report is 34 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##              master     #2609   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity      1824      1839   +15     
===========================================
  Files            184       184           
  Lines           4517      4544   +27     
===========================================
+ Hits            4517      4544   +27     

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