test: add variable system tests and jest config updates

- Add comprehensive tests for variable system, including parsing,
  expression evaluation, and reactivity.
- Add `moduleNameMapper` to `jest.config.js` to resolve `.js` imports
  to `.ts` source files.
- Add a mock for `csv-parse/browser/esm/sync` to support Node-based
  testing.

jest.config.js

@@ -4,6 +4,10 @@ export default {
   testEnvironment: 'node',
   testMatch: ['**/*.test.ts'],
   moduleFileExtensions: ['ts', 'tsx', 'js', 'jsx'],
+  moduleNameMapper: {
+    // Resolve .js imports to .ts source files (ESM convention in TS source)
+    '^(.+)\\.js$': ['$1.ts', '$1.tsx', '$1.js'],
+  },
   transform: {
     '^.+\\.tsx?$': [
       'ts-jest',