fix: missing vitest imports break 3 test files at runtime

[michaeldistel]

3 test files use beforeEach but don't import it from vitest, causing ReferenceError at runtime.

Files:

• tests/semantic/std-function-registry.test.ts
• tests/semantic/type-checker.test.ts
• tests/backend/codegen.test.ts

Fix: add beforeEach to the vitest import in each file.