docs: give subsystems/ its own README index; retire the data-structures-catalog framing

core.md doubled as the folder index: its intro claimed "this folder catalogs the data structures" and carried the 38-row page table, wording that predates the one-page-per-subsystem shape where every page also carries its generated Cordis surface. The folder index now lives in docs/subsystems/README.md (page table plus the type-equiv note), and core.md is one subsystem page among siblings: the spine vocabulary. Structural referents move with it: the docs/AGENTS.md tier table and update rule, development.md's type-equiv pointer, the dsh-code-review skill, the two owning catalog Agent Notes, and website/docs.ts (README projects as reference/subsystems/index.md and takes the docs/subsystems folder alias; sidebar orders shift by one). Remaining "data-structure catalog" / "sub-page" phrasing in active notes and READMEs is reworded to subsystem-page terms in both languages; touched pairs re-recorded; translation-prompt snapshot re-recorded (its example embeds development.md).

docs/development.zh.md

@@ -156,7 +156,7 @@ pnpm run demo:acp
 
 ### 逐字记录类型（`ts type-equiv`）
 
-[核心数据结构](subsystems/core.md)文档会把与源码等价的声明及其原始 JSDoc 一并粘贴，让读者看到确切形状和源码契约。为防止粘贴内容在源码变化时漂移，请将其围栏为 ` ```ts type-equiv `（而不是 ` ```ts `），并在 `scripts/type-equiv.manifest.json` 中登记它镜像的源文件和符号：
+[子系统](subsystems/README.md)页面会把与源码等价的声明及其原始 JSDoc 一并粘贴，让读者看到确切形状和源码契约。为防止粘贴内容在源码变化时漂移，请将其围栏为 ` ```ts type-equiv `（而不是 ` ```ts `），并在 `scripts/type-equiv.manifest.json` 中登记它镜像的源文件和符号：
 
 ```json
 { "doc": "docs/subsystems/session.md", "symbol": "SessionEvent", "source": "packages/core/session/src/types.ts" }