test: expand token utility coverage

[Correxxt]

Closes #9.

This adds focused coverage for token utilities that was missing from tokens.test.ts:

• token order through a nested expression;
• direct iteration of a token node;
• the default source-file path in forEachToken.

Validation:

• pnpm test --run src/tokens.test.ts (5 tests passed)
• ESLint on src/tokens.test.ts
• pnpm tsc
• pnpm build

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