docs: embed the rendered class diagram in the scatter-gather README

Render etc/scatter-gather.urm.puml to PNG and embed it in the detailed explanation section, matching the other modules, instead of the inline mermaid block.

Author: ylcn91

scatter-gather/README.md

@@ -65,6 +65,8 @@ sequenceDiagram
     ScatterGather->>Client: aggregate() returns Harbor Stays 295.50
 ```
 
+![Scatter-Gather class diagram](./etc/scatter-gather.urm.png)
+
 ## Programmatic Example of Scatter-Gather Pattern in Java
 
 The request is a plain immutable value. Every recipient receives exactly the same instance.
@@ -194,86 +196,6 @@ Aggregate phase: choosing the cheapest of 2 quotes
 Best offer: Harbor Stays at 295.50
 ```
 
-## Class Diagram
-
-```mermaid
-classDiagram
-    class RateRequest {
-        +String city
-        +LocalDate checkIn
-        +int nights
-    }
-
-    class RateQuote {
-        +String provider
-        +BigDecimal total
-    }
-
-    class RateProvider {
-        <<interface>>
-        +name() String
-        +quote(RateRequest request) RateQuote
-    }
-
-    class InMemoryRateProvider {
-        -String name
-        -BigDecimal nightlyRate
-        +name() String
-        +quote(RateRequest request) RateQuote
-    }
-
-    class DelayedRateProvider {
-        -RateProvider delegate
-        -Duration delay
-        +name() String
-        +quote(RateRequest request) RateQuote
-    }
-
-    class FailingRateProvider {
-        -String name
-        +name() String
-        +quote(RateRequest request) RateQuote
-    }
-
-    class Aggregator~T, R~ {
-        <<interface>>
-        +aggregate(List~T~ replies) R
-        +cheapestQuote()$ Aggregator
-    }
-
-    class PendingReply {
-        +RateProvider provider
-        +CompletableFuture~RateQuote~ reply
-    }
-
-    class ScatterGather {
-        -ExecutorService executor
-        -Duration timeout
-        +scatter(RateRequest request, List~RateProvider~ providers) List~PendingReply~
-        +gather(List~PendingReply~ pending) List~RateQuote~
-        +scatterGather(RateRequest request, List~RateProvider~ providers, Aggregator aggregator) R
-        +close() void
-    }
-
-    class App {
-        +main(String[] args)$ void
-        ~reportBestOffer(Optional~RateQuote~ best)$ void
-    }
-
-    InMemoryRateProvider ..|> RateProvider
-    DelayedRateProvider ..|> RateProvider
-    FailingRateProvider ..|> RateProvider
-    DelayedRateProvider --> RateProvider : delegates to
-    RateProvider ..> RateRequest : reads
-    RateProvider ..> RateQuote : answers with
-    ScatterGather *-- PendingReply : nested record
-    ScatterGather ..> RateProvider : calls
-    ScatterGather ..> Aggregator : reduces with
-    PendingReply --> RateProvider
-    PendingReply ..> RateQuote
-    App ..> ScatterGather
-```
-
 ## When to Use the Scatter-Gather Pattern in Java
 
 * The same question has to be answered by several independent services, such as price comparison, search federation, or quorum reads.