Add resilience: Spring Framework 7 @Retryable and @ConcurrencyLimit

Companion code for "Spring Framework 7's Built-in Resilience: @Retryable,
@ConcurrencyLimit, and What's Left for Resilience4j". Every retry counted
by recording real invocations: defaults, backoff and jitter, timeout,
reactive and CompletableFuture returns, the concurrency limit's BLOCK and
REJECT policies, retries around transactions, composition with
Resilience4j 2.4.0, and the annotation API across 7.0.0-7.0.9.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01C3TETMrqVUWeFkNtz3Jbo3

resilience/docs/output/retry-mono.txt

@@ -0,0 +1,21 @@
+# GET /demo/retry/mono   (Spring Framework 7.0.9, Resilience4j 2.4.0, JDK 25)
+# Source: src/main/java/com/ankurm/resilience/web/DemoController.java
+
+{
+    "invocations": 3,
+    "started at (ms)": [
+        37,
+        65,
+        75
+    ],
+    "gaps (ms)": [
+        28,
+        10
+    ],
+    "elapsed (ms)": 75,
+    "caller": "returned: ok after 3 invocation(s)",
+    "MethodRetryEvents": [
+        "mono failed with TransientException -> will retry",
+        "mono failed with TransientException -> will retry"
+    ]
+}