[3.x] Add root static file passthrough with new _static/ directory support

[emmadesilva]

Summary

• add an optional _static/ convention that copies files verbatim to the compiled site root while preserving relative paths
• serve _static/ files through the realtime compiler, with _static/media taking precedence over the _media fallback
• preflight collisions against outputs Hyde is producing in the current build while allowing prior passthrough output to be replaced
• document the convention without adding scaffold files or configuration

Deleting a source from _static may leave its previous output because Hyde's existing cleaner does not track historical passthrough mappings.

Tests

• focused static passthrough and realtime compiler tests: 51 passed (159 assertions)
• FeatureFramework, UnitFramework, and FeatureHyde: 3,812 passed; documented PHP 8.5 FeaturedImageUnitTest failure remains; socket test passed separately with loopback access
• php monorepo/HydeStan/run.php: no errors

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d9690af) to head (04c2b13).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##              master     #2585   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity      1807      1815    +8     
===========================================
  Files            183       184    +1     
  Lines           4474      4506   +32     
===========================================
+ Hits            4474      4506   +32     

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