feat: add item opacity and blur

[han4wluc]

Summary

• add opacity and blur support to character and visual items
• persist item appearance across omitted actions and support appearance-only updates with blur clearing
• document the interface and add schema, system, render-state, and VT coverage
• bump package version to 1.19.0 and align Vitest tooling versions for coverage

Testing

• bun run lint
• bun run test
• bun run vitest run --coverage --coverage.reportsDirectory=/tmp/route-engine-coverage-item-opacity-blur
• rtgl vt screenshot --item character/opacity-blur.yaml --item visual/opacity-blur.yaml --wait-event vt:ready --concurrency 1 --timeout 60000
• rtgl vt report --diff-threshold 0.8 --item character/opacity-blur.yaml --item visual/opacity-blur.yaml