Reliable style precedence and validation

Contract change type: semantic change

Fixed

• Established matching CSS layer order in the theme bundle and WordPress
  global styles so component heading sizes win in either stylesheet order,
  while default headings continue to inherit WordPress global styles.
• Made design-system drift fail validation, excluding generated output and
  binary files while allowing Spectre token references and component recipes.
• Made asset validation reject missing files, paths outside the build
  directory, non-file assets, and unresolved imported manifest entries.

Added

• Added validator regression tests to the full check gate and browser-based
  computed heading precedence checks to the WordPress smoke workflow.