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/r4j-retry.txt

@@ -0,0 +1,17 @@
+# GET /demo/r4j/retry   (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)": [
+        7,
+        26,
+        36
+    ],
+    "gaps (ms)": [
+        19,
+        10
+    ],
+    "elapsed (ms)": 37,
+    "caller": "threw: com.ankurm.resilience.support.TransientException: payment gateway 503 (call 3)"
+}