feat(duration): add DurationValue struct for time representation

- Introduces a new `DurationValue` record struct to represent time quantities in specified units.
- Removes the `None` duration unit from the `DurationUnit` enum.
- Enforces non-negative counts in the `DurationValue` constructor.
- Implements string formatting for displaying duration in a user-friendly manner.
- Includes comparison logic to ensure durations of the same unit can be compared.

Author: winromulus

src/ES.FX/Primitives/DurationUnit.cs

@@ -45,11 +45,6 @@ namespace ES.FX.Primitives;
 [PublicAPI]
 public enum DurationUnit
 {
-    /// <summary>
-    ///     No duration unit.
-    /// </summary>
-    None,
-
     /// <summary>
     ///     A 100-nanosecond interval.
     /// </summary>

src/ES.FX/Primitives/DurationValue.cs

@@ -0,0 +1,52 @@
+using JetBrains.Annotations;
+
+namespace ES.FX.Primitives;
+
+/// <summary>
+///   Represents a quantity of time, in a specified unit.
+/// </summary>
+[PublicAPI]
+public readonly record struct DurationValue : IComparable<DurationValue>
+{
+    /// <summary>
+    ///   The number of units.
+    /// </summary>
+    public long Count { get; init; }
+
+    /// <summary>
+    ///   The unit of time for this duration.
+    /// </summary>
+    public DurationUnit Unit { get; init; }
+
+    /// <summary>
+    ///   Initializes a new <see cref="DurationValue"/> with the specified count and unit,
+    ///   enforcing non-negative counts.
+    /// </summary>
+    /// <param name="count">The number of units. Must be non-negative.</param>
+    /// <param name="unit">The duration unit.</param>
+    public DurationValue(long count, DurationUnit unit)
+    {
+        if (count < 0)
+            throw new ArgumentOutOfRangeException(nameof(count), "Count must be non-negative");
+
+        Count = count;
+        Unit = unit;
+    }
+
+    /// <summary>
+    ///   Returns a string such as "7 Days".
+    /// </summary>
+    public override string ToString() => $"{Count} {Unit}{(Count == 1 ? string.Empty : "s")}";
+
+    /// <summary>
+    ///   Compares two durations of the same unit.
+    ///   Comparing different units is not supported.
+    /// </summary>
+    public int CompareTo(DurationValue other)
+    {
+        if (Unit != other.Unit)
+            throw new InvalidOperationException("Cannot compare durations of different units.");
+
+        return Count.CompareTo(other.Count);
+    }
+}