Add client-side custom item creation via server RPCs

[codefling-0xf]

Summary

Adds runtime creation of custom ItemDefinition / ItemBlueprint objects on the client, driven by server RPCs. Lets servers/plugins ship fully client-side items (craftable, with icons) without bundling assets or touching server-side ItemManager.

What's included

CommunityEntity.Items.cs (new)

• CreateItem(JSON.Object[, refresh]) — builds an ItemDefinition (and optional ItemBlueprint) on a fresh GameObject, registers it into ItemManager.itemList / itemDictionary / itemDictionaryByName, and (when a blueprint is present) into bpList / itemToBlueprint / ingredientToBlueprints / defaultBlueprints.
• Server API: SendCreateCustomItems(player, json) / SendRemoveCustomItems(player, json) — send a JSON array over RPC via StringRaw (same pattern as AddUI).
• RPC_CreateCustomItems / RPC_RemoveCustomItems — parse the array, create/remove per entry. UI refresh (LocalPlayer.OnInventoryChanged + UIBlueprints.Refresh) is batched to once per RPC instead of once per item.
• defaultBlueprints is maintained surgically (AddDefaultBlueprint / RemoveDefaultBlueprint) — no full bpList rebuild.
• RemoveCustomItems() cleanup on disconnect via the ClientDisconnect broadcast hook.

CommunityEntity.UI.ServerImage.cs

• Icon sprites are set by CRC through the existing image callback system: a new ItemDefinitionEntry list on ImageRequest (mirrors the existing Entry list). ApplyTextureToItem(def, crc) registers an entry or applies immediately if the texture is cached; CL_ReceiveFilePng applies the sprite to all waiting definitions on arrival. No per-frame polling.

CommunityEntity.cs

• Centralized ClientDisconnect hook (calls RemoveCustomItems()) and the single ParseEnum<T> (moved out of UI.cs to resolve a duplicate-definition conflict).

Example

var items = new JSON.Array();
items.Add(new JSON.Object {
    ["shortname"] = "custom_rpc_item",
    ["displayName"] = "Custom RPC Item",
    ["category"] = ItemCategory.Misc.ToString(),
    ["stackable"] = 100,
    ["rarity"] = Rarity.Uncommon.ToString(),
    ["ingredients"] = new JSON.Array {
        new JSON.Object { ["shortname"] = "wood",   ["amount"] = 100 },
        new JSON.Object { ["shortname"] = "stones", ["amount"] = 50  },
    },
    ["craftTime"] = 5,
    ["workbenchLevelRequired"] = 1,
    ["defaultBlueprint"] = true,
    ["iconSpriteCrc"] = 1234567890u.ToString(),
});
CommunityEntity.ServerInstance.SendCreateCustomItems(player, items.ToString());

Notes

• Items exist only on the client (no server-side ItemManager registration) — intended for client-side UI/crafting features.
• Build has not been tested yet.