test: scaffold Vitest and add initial test suite (100% coverage)

[JoseFredes]

Summary

The repo had no tests and npm test was a placeholder. This PR sets up a test runner and adds an initial suite that brings line/branch/function coverage on src/ to 100%.

• Tooling: Vitest + jsdom + @testing-library/react + @vue/test-utils + @vitest/coverage-v8. New scripts: npm test, npm run test:watch, npm run test:coverage.
• core.ts (test/core.test.ts, 27 tests): input validation (null/undefined/primitives, missing type, the VALID_HTML_TAG regex against attribute-injection attempts like "div onclick=..." and "<script>"), className/style handling, attribute coercion (numeric/boolean/null), event handler wiring + name normalization, and XSS-safe text children.
• Adapters: parity-style tests for the generic, React, and Vue adapters covering rendering, nested children, event wiring, and HTML-escape behavior.
• .gitignore fix: the existing .gitIgnore (capital I) was not matched on Linux, so node_modules/ was effectively unignored and node_modules/.package-lock.json was tracked. Renamed to .gitignore, added coverage/, untracked the stray file.

Coverage

File               | % Stmts | % Branch | % Funcs | % Lines
-------------------|---------|----------|---------|--------
All files          |     100 |      100 |     100 |     100
 core.ts           |     100 |      100 |     100 |     100
 generic-adapter   |     100 |      100 |     100 |     100
 react-adapter     |     100 |      100 |     100 |     100
 vue-adapter       |     100 |      100 |     100 |     100

Follow-ups (not in this PR)

• CI workflow to run npm test on PRs and gate on coverage.
• Cross-adapter parity tests asserting equivalent output from the same JSON across all three.
• Property-based fuzzing of renderComponentFromJSON with fast-check.
• .d.ts drift check (tsd / expect-type) — types/jbricks.d.ts types renderVueComponent as object, while the source returns a defineComponent result.
• Style-key edge case: core.ts assigns style keys via bracket assignment, which silently no-ops for kebab-case keys (e.g. "background-color"); worth deciding whether to support that and adding a regression test.

Test plan

• npm test passes (40/40)
• npm run test:coverage reports 100%
• CI workflow added in a follow-up PR

---

Generated by Claude Code