i18n: Simplified Chinese reads as mainland Chinese, with rows for the strings added since (review of #303)

- Taiwan vocabulary left over from the zh-hant rows #303 started from is
  replaced with the mainland's, about 560 rows: 档案 → 文件, a Taiwan 文件
  (document) → 文档, 工作区域 → 画板, 笔画 → 描边, 色票 → 色板, 遮色片 → 蒙版,
  渐层 → 渐变, 偏好设定 → 首选项, 快速键 → 快捷键, 按一下 → 单击, 描述档 → 配置文件,
  品质 → 质量, 汇出/汇入 → 导出/导入, 连结 → 链接, 点阵 → 栅格, a row 列 → 行,
  「」 → “”, half-width parentheses; tool names that collided or were wrong
  are fixed (Selection Tool 选择工具, Scale Tool 比例缩放工具, Warp 变形,
  Puppet Warp 操控变形, Divide 分割, Exclude 差集, the 直排 type tools).
- 42 rows for strings that landed after #303 (Layers panel, graphics
  processor preference, panel collapse, the font menu's filters).
- A catalog header documenting the format and the vocabulary, and a test
  that zh-hans has no Traditional characters or Taiwan terms.

rows were the better base (hand-adjusted mainland terms in the menus and
dialogs), so no rows were taken from #316.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/development.md

@@ -74,12 +74,10 @@ text at render time from one catalog per language (`i18n/<code>.tsv`; the format
 always use the English ids and labels, so agents and scripts never see translated text.
 
 Languages shipped: English (`en`, the source), Traditional Chinese (`zh-hant`, complete, in the vocabulary used
-in Taiwan; `zh-TW`, `zh-HK`, `zh-MO` and `zh-Hant-*` locales all resolve to it), Japanese (`ja`, complete),
-Czech (`cs`, every menu label) and Brazilian Portuguese (`pt-br`, every menu label and every `tl!` literal).
-Untranslated text falls back to English until its rows are
-added. Simplified Chinese locales
-(`zh-CN`, `zh-SG`, `zh-Hans`) fall back to English until a `zh-hans` catalog is registered: the resolver already
-tells the two scripts apart, so the Traditional catalog is never shown to a Simplified locale.
+in Taiwan; `zh-TW`, `zh-HK`, `zh-MO` and `zh-Hant-*` locales all resolve to it), Simplified Chinese (`zh-hans`,
+complete, in the vocabulary used in mainland China; `zh-CN`, `zh-SG`, `zh-Hans-*` and a bare `zh` resolve to it,
+so the two scripts never mix), Japanese (`ja`, complete), Czech (`cs`, every menu label) and Brazilian Portuguese
+(`pt-br`, every menu label and every `tl!` literal). Untranslated text falls back to English until its rows are added.
 
 - `tl!("…")` translates a literal into the language the UI is drawn in; `i18n::t(s)` is the same for a
   `&str`. `tr(lang, s)` takes the language; `tr_ctx` when one English word needs different translations;