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/01-two-resilience-stacks.md

@@ -0,0 +1,93 @@
+# 1. Two resilience stacks on one classpath
+
+[README](../README.md) | Next: [02-circuit-breaker.md](02-circuit-breaker.md)
+
+Spring Boot 4.1 sits on Spring Framework 7, and Framework 7 shipped something new under
+`org.springframework.resilience`: `@Retryable`, `@ConcurrencyLimit`, and the annotation that
+turns them on, `@EnableResilientMethods`. None of this existed in Framework 6. This repo pins
+down exactly what moved into core, what didn't, and two build breaks you will hit the moment
+you try to wire it up on Boot 4 — both found by running the build, not by reading a changelog.
+
+## What's actually in core (verified by `javap`, not by the reference docs)
+
+`org.springframework.resilience.annotation.Retryable` (an annotation, not the unrelated
+`org.springframework.core.retry.Retryable` *interface* that also ships in `spring-core` — two
+classes with the same simple name in the same major version, easy to import the wrong one):
+
+```
+value(), includes(), excludes(), predicate(),
+maxRetries(), maxRetriesString(),
+timeout(), timeoutString(),
+delay(), delayString(),
+jitter(), jitterString(),
+multiplier(), multiplierString(),
+maxDelay(), maxDelayString(),
+timeUnit()
+```
+
+`org.springframework.resilience.annotation.ConcurrencyLimit`:
+
+```
+value(), limit(), limitString(), policy()   // policy: BLOCK (default) or REJECT
+```
+
+Neither annotation has a `fallbackMethod` attribute. Retrying exhausted just rethrows.
+Rejecting under `REJECT` throws `org.springframework.resilience.InvocationRejectedException`
+(a `java.util.concurrent.RejectedExecutionException` subtype) straight at the caller.
+
+There is no circuit breaker class anywhere in `spring-context-7.0.9.jar` (grepped the whole
+jar listing for `circuitbreaker`, `ratelimit`, `bulkhead` — zero matches). `@ConcurrencyLimit`
+is the closest core has to a Resilience4j Bulkhead, and even that is a decades-old class
+repurposed: `ConcurrencyLimitBeanPostProcessor$ResilienceConcurrencyThrottleInterceptor`
+extends `org.springframework.aop.interceptor.ConcurrencyThrottleInterceptor`, which has shipped
+in Spring since the 1.x era.
+
+## `@EnableResilientMethods` is not automatic
+
+Spring Boot 4.1's autoconfigure jar carries no auto-configuration for the resilience package —
+grepping `spring-boot-autoconfigure-4.1.1.jar`'s listing for "resilien" returns nothing. You
+must put `@EnableResilientMethods` on a `@Configuration` class yourself (this repo puts it on
+the `@SpringBootApplication` class). Skip it and the annotations are inert: no error, no log
+line, the method just runs unprotected.
+
+## Build break #1: `spring-boot-starter-aop` no longer exists
+
+Every pre-Boot-4 Resilience4j guide, including the version of this post it replaces, tells you
+to add `spring-boot-starter-aop`. On Boot 4 that dependency breaks the build outright:
+
+```
+[ERROR] 'dependencies.dependency.version' for org.springframework.boot:spring-boot-starter-aop:jar is missing.
+```
+
+`repo1.maven.org/maven2/org/springframework/boot/spring-boot-starter-aop/maven-metadata.xml`
+confirms it: the last version ever published is `4.0.0-M2`, a milestone. It never reached
+Boot 4.0 GA and was not revived for 4.1. The fix is `org.springframework:spring-aop` directly —
+see [pom.xml](../pom.xml).
+
+## Build break #2 (silent): no `aspectjweaver`, no proxy, no error
+
+Swapping in `spring-aop` alone gets you a clean build and a **wrong result**. Resilience4j's
+Spring integration (`resilience4j-spring6`, which is what `resilience4j-spring-boot4` actually
+depends on — see below) implements `CircuitBreakerAspect`, `BulkheadAspect`, `RateLimiterAspect`
+and friends as real `@Aspect` classes (confirmed with `unzip -l` on the jar). Spring's
+`AnnotationAwareAspectJAutoProxyCreator` needs `org.aspectj:aspectjweaver` on the classpath to
+even recognise a bean as an aspect. Without it, **zero proxies get created, for anything** —
+Resilience4j's annotations and Spring's own `@Retryable` both go completely inert, silently.
+
+That is exactly what happened building this repo: `CircuitBreakerTripAndRecoverTest` failed
+with the raw `DownstreamUnavailableException` propagating straight out of `R4jPaymentService`,
+no fallback, no state tracking — because there was no proxy in front of it at all. Adding
+`org.aspectj:aspectjweaver` fixed every failing test in the same run. `spring-boot-starter-aop`
+used to bundle this for you; its replacement doesn't, and nothing tells you that.
+
+## A naming trap in the dependency itself
+
+`resilience4j-spring-boot4` is a real, separate artifact from `resilience4j-spring-boot3`
+(both currently at `2.4.0`) — but its own POM depends on `resilience4j-spring6`, `spring-core
+7.0.2`, `spring-context 7.0.2` and `spring-boot-autoconfigure 4.0.0`. The "spring-boot4" in the
+artifact name is about which *Boot* generation it targets, not which internal Resilience4j
+module version it's built on — that module never got renamed to "spring7". If you're grepping
+your dependency tree for "spring7" expecting to find the pieces Boot 4.1 pulls in, you won't.
+
+Next: [02-circuit-breaker.md](02-circuit-breaker.md) — the Resilience4j side, verified against
+a real trip/recover run.