docs: Deduplicate validate-check helper

Keep a single Makefile validate-check target that prints the full
/metrics dump, highlights hit/miss fields, and curls /lancache-heartbeat.
Drop the duplicate README troubleshooting item and align the validation
chapter plus validate-config.yaml comments with that behavior.

Link: #29

Makefile

@@ -62,15 +62,7 @@ validate run-validation: build clean-disk ## Start steamcache2 on :80 with small
 	fi; \
 	exec "$$BINARY" --config docs/examples/validate-config.yaml --log-level info
 
-validate-check: ## Print hit/miss fields from http://localhost/metrics (default listen_address :80)
-	@echo "=== steamcache2 cache check (http://localhost/metrics) ==="
-	@curl -sS --fail --max-time 5 http://localhost/metrics | grep -E '^(total_requests|cache_hits|cache_misses|hit_rate|memory_cache_hits|disk_cache_hits|errors) ' || { \
-		echo "ERROR: could not read hit/miss metrics from http://localhost/metrics"; \
-		echo "Is steamcache2 running on the default listen address (:80)?"; \
-		exit 1; \
-	}
-
-validate-check: ## Curl local /metrics (hit/miss) and /lancache-heartbeat (default :80)
+validate-check: ## Curl local /metrics (full dump + hit/miss fields) and /lancache-heartbeat (default :80)
 	@echo "=== http://localhost/metrics ==="
 	@metrics=$$(curl -sf --max-time 5 http://localhost/metrics) || { \
 		echo "ERROR: could not fetch http://localhost/metrics"; \
@@ -82,9 +74,15 @@ validate-check: ## Curl local /metrics (hit/miss) and /lancache-heartbeat (defau
 	echo "=== hit/miss fields ==="; \
 	printf '%s\n' "$$metrics" | grep -E '^(total_requests|cache_hits|cache_misses|hit_rate|memory_cache_hits|disk_cache_hits|errors) ' || true; \
 	echo ""; \
-	echo "=== http://localhost/lancache-heartbeat (expect 204 + X-LanCache-Processed-By: SteamCache2) ==="; \
-	curl -sD - -o /dev/null --max-time 5 http://localhost/lancache-heartbeat || { \
+	echo "=== http://localhost/lancache-heartbeat (GET; expect 204 + X-LanCache-Processed-By: SteamCache2) ==="; \
+	hb=$$(curl -sD - -o /dev/null --max-time 5 http://localhost/lancache-heartbeat) || { \
 		echo "ERROR: could not fetch http://localhost/lancache-heartbeat"; \
+		echo "Is steamcache2 running on the default listen address :80?"; \
+		exit 1; \
+	}; \
+	printf '%s\n' "$$hb"; \
+	echo "$$hb" | grep -q '204' && echo "$$hb" | grep -qi 'X-LanCache-Processed-By' || { \
+		echo "ERROR: expected HTTP 204 and X-LanCache-Processed-By on /lancache-heartbeat"; \
 		exit 1; \
 	}
 
@@ -135,7 +133,7 @@ help: ## Show this help message
 	@echo "  clean-disk         Remove disk cache"
 	@echo "  bench              Run low-level VFS microbenchmarks"
 	@echo "  validate / run-validation  Start server on :80 (builds, auto-setcaps fresh binary, then runs as normal user, cleans disk cache first)"
-	@echo "  validate-check     Print hit/miss fields from http://localhost/metrics (default listen :80)"
+	@echo "  validate-check     Curl local /metrics (full dump + hit/miss fields) and /lancache-heartbeat (default :80)"
 	@echo "  setcap               Explicitly set cap on current build (for port 80 use outside validate)"
 	@echo "  validate-kill      Kill leftover steamcache2 processes (safer)"
-	@echo "  prefill            Download latest SteamPrefill into bin/steam-prefill/SteamPrefill (gitignored)"
+	@echo "  prefill            Download latest SteamPrefill into bin/steam-prefill/SteamPrefill (gitignored)"