Remove unused `gcd()` export from `mathUtil.ts` — no production consumer

## Recommendation

### 🗑️ SAFE TO REMOVE

`src/utils/mathUtil.ts` exports `gcd()` but it's only imported in `mathUtil.test.ts`. Only `lcm()` has a production consumer (via `nodeDefUtil.ts`).

## How to Fix

1. Verify: `grep -r 'gcd' src/ --include='*.ts' --include='*.vue' | grep -v test | grep -v '.d.ts'`
2. Remove the export (keep as internal if `lcm` depends on it, or inline)

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**Category:** Dead Code | **Priority:** Nice-to-have | **Effort:** Trivial
Part of #11022
_Filed by repo-audit skill_

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-11082-Remove-unused-gcd-export-from-mathUtil-ts-no-production-consumer-33e6d73d365081059313c7c75230e386) by [Unito](https://www.unito.io)