taxonomy: add zh-CN translations for "Composite foods" subtree (4 entries, human-reviewed) (#13902)

Adds Simplified Chinese (zh-CN, mainland) translations to the
`Composite foods` subtree in `taxonomies/food_groups.txt`.

4 entries translated:
- Composite foods -> 复合食品
- Pizza pies and quiches -> 比萨和法式咸派
- One-dish meals -> 单品套餐
- Sandwiches -> 三明治

## Background

This PR is part of the Agentic Commons taxonomy translation pilot,
discussed with OFF maintainers in

[contributor-quality-issues#21](openfoodfacts/contributor-quality-issues#21)
and follow-up threads on the OFF Slack (#general / #quality-data /
#reuse). Maintainers indicated taxonomy translation is a reasonable
agentic direction provided each PR is paced, human-reviewed, and the
changelog is functional rather than promotional. We are starting
small (one subtree per PR, controlled cadence) so reviewers can
judge quality before any scaling decision.

## Translation methodology

- LLM-generated by an agent, schema-validated against the existing entry
structure.
- Mainland-China vocabulary; no traditional characters; no Taiwan/HK
regional terms
  (e.g. 果干 not 干果, 蔬菜 not 青菜).
- **Manually reviewed by a Simplified-Chinese-native maintainer**
  at [agentic-commons.org](https://agentic-commons.org) before being
  marked ready for review.

## Source / contact

- Project: [agentic-commons.org](https://agentic-commons.org)
- Contact: git@agentic-commons.org

---
Submission marker: sm_1wfy213e

Author: agentic-commons-bot

taxonomies/food_groups.txt

@@ -389,16 +389,20 @@ fr: Sauces
 ja: ソース
 
 en: Composite foods
+zh: 复合食品, 混合食品, 组合食品
 
 < en: Composite foods
 en: Pizza pies and quiches
+zh: 比萨和法式咸派, 披萨和乳蛋饼, 比萨饼和法式蛋饼
 
 < en: Composite foods
 en: One-dish meals
+zh: 单品套餐, 一锅料理, 单锅菜
 
 < en: Composite foods
 en: Sandwiches
 ja: サンドイッチ
+zh: 三明治
 
 en: Beverages
 fr: Boissons