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/02-circuit-breaker.md

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+# 2. The circuit breaker, verified
+
+[Previous: 01-two-resilience-stacks.md](01-two-resilience-stacks.md) | [README](../README.md) | Next: [03-spring-retryable.md](03-spring-retryable.md)
+
+Source: [`R4jPaymentService.java`](../src/main/java/com/ankurm/resilience/r4j/R4jPaymentService.java).
+Config: [`application.yml`](../src/main/resources/application.yml).
+Test: [`CircuitBreakerTripAndRecoverTest.java`](../src/test/java/com/ankurm/resilience/r4j/CircuitBreakerTripAndRecoverTest.java).
+Transcript: [`docs/output/01-circuitbreaker-trip.txt`](output/01-circuitbreaker-trip.txt).
+
+## The state machine
+
+```
+CLOSED --(failure rate >= threshold, over >= minimumNumberOfCalls)--> OPEN
+OPEN --(waitDurationInOpenState elapses)--> HALF_OPEN
+HALF_OPEN --(permittedNumberOfCallsInHalfOpenState calls all succeed)--> CLOSED
+HALF_OPEN --(a probe call fails)--> OPEN
+```
+
+This repo's config: `slidingWindowSize=10`, `minimumNumberOfCalls=5`, `failureRateThreshold=50`,
+`waitDurationInOpenState=2s`, `permittedNumberOfCallsInHalfOpenState=2`.
+
+## What the transcript actually shows
+
+Calls 1-5 against a dead downstream: each one really calls the downstream, gets a
+`DownstreamUnavailableException`, and the fallback method runs. By call 5 the breaker has seen
+5 calls (the configured minimum) at a 100% failure rate and flips to `OPEN`. Call 6 is the
+first call that does **not** touch the downstream at all — it gets `CallNotPermittedException`
+instead, straight from the breaker, before `R4jPaymentService.pay()`'s body ever runs.
+
+Calls 7-9 (phase 2 in the transcript) confirm the downstream call counter is frozen at 5 for
+the rest of the OPEN period — three more calls, zero more downstream traffic. This is the
+property `@Retryable` cannot offer on its own (see [chapter 3](03-spring-retryable.md)): a
+circuit breaker remembers that the dependency is down and stops asking.
+
+After `waitDurationInOpenState` (2s; the test waits 2.2s to be safely past it) the breaker
+allows exactly `permittedNumberOfCallsInHalfOpenState` probe calls through. The downstream was
+reconfigured to succeed by then, both probes pass, and the breaker closes.
+
+## `minimumNumberOfCalls` and why it exists
+
+A fresh breaker with one failed call and no `minimumNumberOfCalls` floor would trip on pure
+noise — one deployment-time connection refused, one cold JVM. `minimumNumberOfCalls=5` here
+means the first four failures cannot trip anything by themselves; the breaker needs a real
+sample before it judges the dependency.
+
+## Actuator health vs the circuitbreakers endpoint
+
+With `management.endpoint.health.show-details: always`, `GET /actuator/health` reports only an
+aggregate `"circuitBreakers":{"status":"UNKNOWN"}` in this version — no per-instance detail.
+The full breakdown (`state`, `failureRate`, `bufferedCalls`, `notPermittedCalls`, ...) is at
+`GET /actuator/circuitbreakers` instead. Both captured live, breaker actually OPEN, via
+[`ActuatorHealthTest.java`](../src/test/java/com/ankurm/resilience/r4j/ActuatorHealthTest.java):
+[`05-actuator-health-tripped.txt`](output/05-actuator-health-tripped.txt).
+
+- Resilience4j reference: [CircuitBreaker](https://resilience4j.readme.io/docs/circuitbreaker) (`rel=nofollow`)
+- The actual states and transition rules, from the CircuitBreaker interface itself, are worth
+  reading directly rather than from a diagram: `io.github.resilience4j.circuitbreaker.CircuitBreaker.State`.
+
+Next: [03-spring-retryable.md](03-spring-retryable.md).