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.

main.go

@@ -632,7 +632,12 @@ func main() {
 	// instead of being cut off when the process exits.
 	shutdownManager.RegisterComponent("http-proxy", shutdownHTTPProxy)
 
-	// Cache shutdown is handled internally by the cache implementation
+	// Close the Redis cache client's connection pool at shutdown (in-memory
+	// caches are no-ops).
+	shutdownManager.RegisterComponent("cache", func(context.Context) error {
+		libpack_cache.Shutdown()
+		return nil
+	})
 
 	// Start monitoring server
 	cfg.Logger.Info(&libpack_logging.LogMessage{