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/docs/output/03a-retryable-recovers.txt

@@ -0,0 +1,6 @@
+@Retryable(maxRetries=3, delay=200ms, multiplier=2.0): recovering from 2 transient failures
+===========================================================================================
+downstream configured to fail its first 2 calls, then succeed
+result: OK (call #3)
+downstream was actually called 3 times
+elapsed: ~601ms (expect >= 200ms delay before the 2nd attempt, plus backoff before the 3rd)