An extension method that shapes separated Arabic letters.

ArabicShaper.cs

using System.Text;

const string original = "سماء عماد لها وارض تسير وفوقها قمم الروابي فضاء لا انتهاء له وشمس تضيء بحسبة بين السحاب فشد كيانك الادنى برب تنال بقربه شرف الجناب!";

Console.OutputEncoding = Encoding.UTF8;
Console.WriteLine($"Before: \"{original}\", \nafter: \"{original.ShapeArabic()}\".");

internal static class StringExtensions
{
    // Taken from https://en.wikipedia.org/wiki/Arabic_script_in_Unicode#Contextual_forms.
    private static readonly Dictionary<char, (char Final, char Middle, char Initial)> Letters = new()
    {
        { 'ا', ('ﺎ', '\0', '\0') },
        { 'ب', ('ﺐ', 'ﺒ', 'ﺑ') },
        { 'ت', ('ﺖ', 'ﺘ', 'ﺗ') },
        { 'ث', ('ﺚ', 'ﺜ', 'ﺛ') },
        { 'ج', ('ﺞ', 'ﺠ', 'ﺟ') },
        { 'ح', ('ﺢ', 'ﺤ', 'ﺣ') },
        { 'خ', ('ﺦ', 'ﺨ', 'ﺧ') },
        { 'د', ('ﺪ', '\0', '\0') },
        { 'ذ', ('ﺬ', '\0', '\0') },
        { 'ر', ('ﺮ', '\0', '\0') },
        { 'ز', ('ﺰ', '\0', '\0') },
        { 'س', ('ﺲ', 'ﺴ', 'ﺳ') },
        { 'ش', ('ﺶ', 'ﺸ', 'ﺷ') },
        { 'ص', ('ﺺ', 'ﺼ', 'ﺻ') },
        { 'ض', ('ﺾ', 'ﻀ', 'ﺿ') },
        { 'ط', ('ﻂ', 'ﻄ', 'ﻃ') },
        { 'ظ', ('ﻆ', 'ﻈ', 'ﻇ') },
        { 'ع', ('ﻊ', 'ﻌ', 'ﻋ') },
        { 'غ', ('ﻎ', 'ﻐ', 'ﻏ') },
        { 'ف', ('ﻒ', 'ﻔ', 'ﻓ') },
        { 'ق', ('ﻖ', 'ﻘ', 'ﻗ') },
        { 'ك', ('ﻚ', 'ﻜ', 'ﻛ') },
        { 'ل', ('ﻞ', 'ﻠ', 'ﻟ') },
        { 'م', ('ﻢ', 'ﻤ', 'ﻣ') },
        { 'ن', ('ﻦ', 'ﻨ', 'ﻧ') },
        { 'ه', ('ﻪ', 'ﻬ', 'ﻫ') },
        { 'و', ('ﻮ', '\0', '\0') },
        { 'ي', ('ﻲ', 'ﻴ', 'ﻳ') },
        { 'ﺁ', ('ﺂ', '\0', '\0') },
        { 'ة', ('ﺔ', '\0', '\0') },
        { 'ى', ('ﻰ', '\0', '\0') }
    };

    /// <summary>
    /// Shapes separated Arabic letters into words.
    /// </summary>
    /// <example>
    /// Shapes text like: "ﻝﻩﺱ" to: "سهل".
    /// </example>
    /// <remarks>
    /// Letters that have vocalization (تشكيل) will not be shaped correctly.
    /// </remarks>
    /// <param name="original">The original, unshaped Arabic <see cref="string"/>.</param>
    /// <returns>The fully shaped Arabic <see cref="string"/>.</returns>
    public static string ShapeArabic(this string original)
    {
        if (original.Length < 2)
        {
            return original;
        }

        return string.Create(
            original.Length,
            original,
            (span, @string) =>
            {
                var reversed = original.Length - 1;

                for (var index = 0; index < @string.Length; index++)
                {
                    var current = @string[reversed];

                    if (!Letters.TryGetValue(current, out var value))
                    {
                        span[index] = current;
                        reversed--;
                        continue;
                    }

                    // Try to find if neighbouring letters have initial/final shapes.
                    // We use that to construct the final shape of the letter in the "middle".
                    var initial = index - 1 >= 0
                                  && Letters.TryGetValue(@string[reversed + 1], out var right)
                                  && right.Final is not '\0';

                    var final = @string.Length > index + 1
                                && Letters.TryGetValue(@string[reversed - 1], out var left)
                                && left.Initial is not '\0';

                    // Try to construct the shape of the letter and count for all shape possibilities.
                    // And in case of failure, always fallback to the nearest "shape".
                    span[index] = (initial, final) switch
                    {
                        (true, true) => value.Middle is '\0' ? value.Final is '\0' ? value.Initial : value.Final : value.Middle,
                        (true, false) => value.Initial is '\0' ? current : value.Initial,
                        (false, true) => value.Final is '\0' ? current : value.Final,
                        _ => current
                    };

                    reversed--;
                }
            });
    }
}

Updated benchmark with 5K characters.

// * Summary *

BenchmarkDotNet v0.13.12, Windows 11 (10.0.22621.3447/22H2/2022Update/SunValley2)
AMD Ryzen 3 3100, 1 CPU, 8 logical and 4 physical cores
.NET SDK 8.0.204
  [Host]     : .NET 8.0.4 (8.0.424.16909), X64 RyuJIT AVX2
  DefaultJob : .NET 8.0.4 (8.0.424.16909), X64 RyuJIT AVX2


| Method      | Mean     | Error    | StdDev   | Gen0   | Allocated |
|------------ |---------:|---------:|---------:|-------:|----------:|
| ShapeArabic | 77.05 us | 0.327 us | 0.306 us | 2.3193 |   9.86 KB |