Add experimental Servo rendering backend

[victor-fdez]

Summary

• add opt-in Servo and Playwright frame capture backends with automatic operational fallback
• add cached render animate_it: PNG responses backed by strict props and opaque render tickets
• add the localhost Rust Servo worker, cancellation/progress protocol, parity verification, and opt-in compatibility workflow
• prepare the public API as AnimateIt 0.6.0 while keeping Chromium as the default

Verification

• bundle exec rspec --exclude-pattern spec/rendering_spec.rb (142 examples)
• RUN_RENDER_SMOKE=1 bundle exec rspec spec/rendering_spec.rb (6 examples)
• bundle exec rubocop
• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo test (12 tests)
• cargo check
• built and inspected the 0.6.0 gem package
• exercised real Servo 0.4.0 single-frame and streamed batch captures locally

Rollout

No composition is automatically eligible. Hosts must explicitly call servo_compatible!, and production deployment remains deferred until visual parity, latency, resource interception, packaging, and supervision gates pass.