feat: add active descendant focus matchers

[gnapse]

Summary

• add toHaveActiveDescendant and toHaveVirtualFocus matchers for aria-activedescendant focus patterns
• document both matchers and add TypeScript declarations
• cover runtime behavior, stale references, duplicate ids, and type usage

Test plan

• npm test -- --watch=false --runInBand --runTestsByPath src/tests/to-have-active-descendant.js
• npm run lint
• npm run test:types
• env FORCE_COLOR=true npm test -- --watch=false --runInBand
• pre-commit validate hook