docs: rename core-data-structures/ to subsystems/

The folder is becoming the home of one-doc-per-subsystem pages (intro +
data structures + cordis services/events), so the name must describe the
whole contract, not just the type-vocabulary third of it. Mechanical
rename rebuilt on current master: every inbound Markdown link, generator
constant, website route, type-equiv manifest path, and spec expectation
moves together; the zh sides of the notes whose prose names the folder
are aligned (子系统) in the same change; touched bilingual pairs
re-recorded; translation-prompt snapshot re-recorded (its example embeds
development.md). Historical Agent Note slugs keep their dated
filenames.

packages/core/session/README.md

@@ -56,7 +56,7 @@ The shared [storage codec](src/chunk-rows.ts) losslessly converts event sequence
 
 ### Surface types
 
-This package owns ordered surface projection, replacement validation, replay, and the type guards that distinguish append-origin from replacement events. The [surface type catalog](../../../docs/core-data-structures/session.md#surface-types) owns the exact shapes and field semantics. A human transcript must project append-origin events rather than `session.surface`, because landed replacements shadow history the reader already saw; model-facing consumers continue to read `session.surface`.
+This package owns ordered surface projection, replacement validation, replay, and the type guards that distinguish append-origin from replacement events. The [surface type catalog](../../../docs/subsystems/session.md#surface-types) owns the exact shapes and field semantics. A human transcript must project append-origin events rather than `session.surface`, because landed replacements shadow history the reader already saw; model-facing consumers continue to read `session.surface`.
 
 ### Request-header reconstruction (`request-header.ts`)