README: add entities loop diagram

Author: harunou

README.md

@@ -259,6 +259,11 @@ A page that needs no server-prepared data skips the split entirely: the sign-in
 `page.tsx` is a plain client component — there the page slot is occupied
 directly by the user interface unit, with no template or template action.
 
+![Next.js entities loop](docs/ca-nextjs-entities-loop.svg)
+
+<details>
+  <summary>mermaid</summary>
+
 ```mermaid
 graph TD
 
@@ -267,7 +272,7 @@ T["Template (server component)"]
 A["Template action"]
 CORE["Server core"]
 CU["Client units (client components)"]
-GW["Gateway (Server Actions)"]
+GW["Gateway (Server Action)"]
 
 R -- "executes" --> T
 T -- "awaits" --> A
@@ -278,6 +283,8 @@ GW -- "drives, via BFF controller" --> CORE
 GW -- "revalidatePath() restarts the loop" --> R
 ```
 
+</details>
+
 ## Gateway/Driver
 
 On the main diagram, the client–server integration boundary contains a single