Refactor runner lifecycle + isolator APIs, and update README

[ngan]

Summary

• refactor the fixture execution model around a Runner that owns configuration + registry state
• rename generation wrappers from Generator to Isolator (FixtureKit::Isolator, FixtureKit::RSpec::Isolator, FixtureKit::TestCase::Isolator)
• replace the old definition context with FixtureKit::Definition, and keep FixtureKit.define returning a definition object
• switch SQL capture to SqlSubscriber.capture using ActiveSupport::Notifications.subscribed
• update fixture registration/mount flow in RSpec integration and keep suite pregeneration in before(:suite)
• rename callback config from on_generate to on_cache (receives fixture name)
• refresh README to document current configuration and lifecycle (isolator, on_cache, preserve-cache behavior)

Notable API Changes

• config.generator -> config.isolator
• FixtureKit::Generator -> FixtureKit::Isolator
• config.on_generate -> config.on_cache
• removed stale autogenerate configuration/docs

Verification

• bundle exec rspec --fail-fast
  • 61 examples, 0 failures