test: cover host toolbar contract

[ddebowczyk]

Summary

• prove an explicit empty toolbar has no default compact or window controls
• prove a host can replace a built-in window control while keeping its controller operation

Verification

• npm test
• npm run lint
• npm run build:lib
• npm run typecheck:consumer
• npm run typecheck:tauri-example
• npm run check:package
• npm pack --dry-run
• markdownlint README.md "docs/**/*.md"
• yq . .github/workflows/ci.yml
• git diff --check