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).

scripts/project-doc-site.spec.ts

@@ -269,14 +269,14 @@ describe('docsPages locale routes', () => {
     }
   })
 
-  it('projects translated core-data pages while retaining explicit English fallbacks', () => {
+  it('projects translated subsystem pages while retaining explicit English fallbacks', () => {
     const rootPages = docsPages.filter(page => (
       page.locale === 'root' && page.route.startsWith('reference/subsystems/')
     ))
     const translated = rootPages.filter(page => page.contentLocale === 'zh-CN')
     const fallbacks = rootPages.filter(page => page.contentLocale === 'en-US')
 
-    expect(translated).toHaveLength(37)
+    expect(translated).toHaveLength(39)
     expect(translated.every(page => page.source.endsWith('.zh.md'))).toBe(true)
     expect(fallbacks.map(page => page.source).sort()).toEqual([
       'docs/subsystems/commands.md',