Add resilience4j-circuit-breaker: Resilience4j 2.4.0 vs Spring Framework 7 core, on Boot 4.1

Companion module for the rewritten post 'Resilience4j Circuit Breaker in Spring Boot 4.1:
What It's Still For', reworked around Framework 7 now shipping @Retryable/@ConcurrencyLimit
in core. Covers what's still Resilience4j's job (circuit breaker, rate limiter, bulkhead's
bounded wait, fallback methods, Actuator/Micrometer metrics), the off-by-one between
maxAttempts and maxRetries, and two Boot-4.1 build breaks: spring-boot-starter-aop no longer
exists (renamed to spring-boot-starter-aspectj, proven with Maven Central metadata and the
renamed starter's own POM -- see resilience/docs/08-starter-aop-renamed-to-starter-aspectj.md
in this same repo), and the resulting fix uses that renamed starter directly rather than
assembling spring-aop + aspectjweaver by hand. Kept as its own module rather than a new
top-level repository, alongside the existing resilience/ module for the sibling Framework-7
post.

resilience4j-circuit-breaker/pom.xml

@@ -0,0 +1,70 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+
+  <parent>
+    <groupId>org.springframework.boot</groupId>
+    <artifactId>spring-boot-starter-parent</artifactId>
+    <version>4.1.1</version>
+    <relativePath/>
+  </parent>
+
+  <groupId>com.ankurm</groupId>
+  <artifactId>resilience-boot4-demo</artifactId>
+  <version>1.0.0</version>
+  <name>resilience-boot4-demo</name>
+  <description>Resilience4j vs Spring Framework 7's built-in @Retryable/@ConcurrencyLimit, on Spring Boot 4.1</description>
+
+  <properties>
+    <java.version>25</java.version>
+    <resilience4j.version>2.4.0</resilience4j.version>
+  </properties>
+
+  <dependencies>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-web</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-actuator</artifactId>
+    </dependency>
+    <!-- spring-boot-starter-aop was RENAMED, not removed. Maven Central's maven-metadata.xml for
+         spring-boot-starter-aop stops at 4.0.0-M2; spring-boot-starter-aspectj's starts at
+         4.0.0-M3 and carries through 4.0.0 GA to 4.1.1 and beyond. Its published POM (verified via
+         repo1.maven.org/.../spring-boot-starter-aspectj/4.1.1/spring-boot-starter-aspectj-4.1.1.pom)
+         depends on exactly org.springframework:spring-aop:7.0.9 and org.aspectj:aspectjweaver:1.9.25.1,
+         the same two jars this file used to add by hand. Adding the old starter name breaks the
+         build (dependencies.dependency.version ... is missing) because that artifact ID is gone
+         from spring-boot-dependencies' BOM; the fix is the renamed starter, one line, version-managed
+         by the parent POM like any other starter. See docs/01-two-resilience-stacks.md. -->
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-aspectj</artifactId>
+    </dependency>
+
+    <!-- Resilience4j, Boot-4-targeted artifact (still built on resilience4j-spring6 internally) -->
+    <dependency>
+      <groupId>io.github.resilience4j</groupId>
+      <artifactId>resilience4j-spring-boot4</artifactId>
+      <version>${resilience4j.version}</version>
+    </dependency>
+
+    <dependency>
+      <groupId>org.springframework.boot</groupId>
+      <artifactId>spring-boot-starter-test</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.springframework.boot</groupId>
+        <artifactId>spring-boot-maven-plugin</artifactId>
+      </plugin>
+    </plugins>
+  </build>
+</project>