Refactor isolators into top-level classes

[ngan]

Summary

• move framework-specific isolators into lib/fixture_kit/isolators as FixtureKit::RSpecIsolator and FixtureKit::MinitestIsolator
• update configuration/autoload wiring and all references in specs/helpers/docs
• simplify isolator failure handling and remove unused custom error classes
• keep harness runs excluded from suite totals and add integration output assertions for expected test counts

Testing

• bundle exec rspec --fail-fast
• FIXTURE_KIT_INTEGRATION_FRAMEWORK=minitest bundle exec rspec --fail-fast