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/04a-concurrencylimit-block.txt

@@ -0,0 +1,7 @@
+@ConcurrencyLimit(limit=2, policy=BLOCK), 4 concurrent callers, each sleeps 300ms
+=================================================================================
+per-caller completion time (ms), sorted: [300, 300, 598, 600]
+total wall time for all 4 callers: 601ms
+all 4 calls succeeded (BLOCK never rejects): true
+expectation: with limit=2 and 300ms per call, 4 callers must take roughly 2x300=600ms+,
+not ~300ms as they would with no limit at all.