fix: restore tracing endpoint validation; honor per-side client timeouts; cache eviction target; redis close

- tracing: validateEndpoint used fmt.Sscanf("%s:%d") which treats %s as
  greedy, so every valid host:port endpoint failed and traced==nil disabled
  tracing entirely. Validate with net.SplitHostPort (pure helper, port range
  0-65535); test TestValidateEndpoint.
- proxy: createFasthttpClient used clientTimeout for every timeout slot,
  ignoring the parsed CLIENT_READ_TIMEOUT/CLIENT_WRITE_TIMEOUT (silent
  config loss, documented separately in README). Honour per-side read/write
  timeouts, falling back to client timeout when unset.
- cache/memory: evictToFreeMemory capped candidates at maxCacheSize/5 and
  stopped at the subset end, so a large write could leave the cache over
  its memory limit. Collect all entries and sort.Slice (correct + O(n log n)
  vs O(cap^2)), evict until the target is met or the cache is drained.
- cache/redis: the Redis cache client was never closed at shutdown (unlike
  the metrics aggregator). Add RedisConfig.Close, CacheWrapper.Close, a
  cache.Shutdown and register it with the ShutdownManager.

cache/cache.go

@@ -168,6 +168,16 @@ func EnableCache(cfg *CacheConfig) {
 	config = cfg
 }
 
+// Shutdown closes the underlying cache client if it owns external resources
+// (the Redis connection pool). No-op for in-memory caches.
+func Shutdown() {
+	if config != nil && config.Client != nil {
+		if closer, ok := config.Client.(io.Closer); ok {
+			_ = closer.Close()
+		}
+	}
+}
+
 func CacheLookup(hash string) []byte {
 	if !IsCacheInitialized() {
 		return nil