Adds a node-side harness that drives the engine wasm directly, fixtures
that build PDFs in-memory, and cases for font fallback and flattening.
The engine loader now tolerates a runtime without import.meta.resolve so
the suite can load it outside a browser.
- Add vitest configuration with coverage reporting
- Create test setup file with DOM mocks
- Add tests for state management, helpers, and tool configurations
- Update tsconfig and package.json for testing support
- Clean up unused comments and improve code style
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