Gestorage v1.0.1-1.21.1

centralize constants, cache shulker detection, unify inventory creation

• Add ModConstants.java to replace all hardcoded size values (27/54/228)
• Add EnderChestFactory to unify EnhancedEnderChestInventory creation
• Add ClientState for cached client-side mode storage
• Cache shulker box detection in ShulkerRefillManager (IdentityHashMap)
• Add defensive clamping in ModGameRules.getEnderChestSize()
• Protect ShulkerLinkManager against double load
• Implement EnderSizeChangedS2CPacket handler
• Document OpenEnderScreenS2CPacket as reserved placeholder
• Remove dead BATCH_SIZE constant from ExtraLargeEnderMenu