feat(engine): add file-role hints and suppress low-signal test fixture noise

[jcouture]

Summary

• Introduces conservative file-role classification (locale_data, test_fixture,
  build_release) based on path/filename conventions
• Suppresses low-signal invisible-character findings in ordinary test source when
  they appear in benign contexts (strings, comments, whitespace, prose) with no nearby
  decode/exec markers
• Downgrade isolated invisible characters in locale data files to LOW severity
• Hardens build/release and shell-sensitive contexts against false suppression —
  bidi controls, payloads, correlations, long runs, and testdata fixtures are never
  softened
• Refactors hasNearbyDecoderMarker into a reusable boolean helper with
  configurable distance