docs: Add README operator cache check

The README buried hit/miss verification under the SteamPrefill validation
chapter, and advertised make validate-check without a Makefile target.
Operators need a short start → traffic → metrics path against the existing
/metrics and /lancache-heartbeat endpoints.

Link: #29

docs/examples/validate-config.yaml

@@ -27,7 +27,9 @@
 #   SteamPrefill benchmark run -c 20 ...
 #
 # After the benchmark run, inspect with:
-#   curl -s http://localhost/metrics
+#   make validate-check
+#   # or: curl -s http://localhost/metrics
+# Heartbeat (process up): curl -s -i http://localhost/lancache-heartbeat
 #
 # Tweak sizes upward if you want to run very large workloads while still
 # exercising the disk tier (workload >> RAM is ideal for real disk testing).