Fix vanilla spell casting and bag contents

Vanilla CMSG_CAST_SPELL target mask is uint16 (not uint32 like WotLK),
the extra 2 bytes were corrupting packets. Also implement full bag
content tracking: extract container slot GUIDs from CONTAINER update
objects, set proper bag sizes, and populate bag items in inventory
rebuild.

include/game/update_field_table.hpp

@@ -52,6 +52,10 @@ enum class UF : uint16_t {
     // Item fields
     ITEM_FIELD_STACK_COUNT,
 
+    // Container fields
+    CONTAINER_FIELD_NUM_SLOTS,
+    CONTAINER_FIELD_SLOT_1,
+
     COUNT
 };