feat(69): auto-tune channel-number allocator

Refs #69

ErsatzTV.Application/Channels/AutoTuneNumberAllocator.cs

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+using System.Globalization;
+
+namespace ErsatzTV.Application.Channels;
+
+public static class AutoTuneNumberAllocator
+{
+    // Allocate `count` sequential integer channel numbers starting at `startingNumber`,
+    // skipping any number already present in `existingNumbers`. Channel.Number is a string,
+    // so numbers are returned as invariant-culture strings.
+    public static List<string> Allocate(int startingNumber, int count, ISet<string> existingNumbers)
+    {
+        var result = new List<string>(count);
+        int next = startingNumber;
+        while (result.Count < count)
+        {
+            string candidate = next.ToString(CultureInfo.InvariantCulture);
+            if (!existingNumbers.Contains(candidate))
+            {
+                result.Add(candidate);
+            }
+
+            next++;
+        }
+
+        return result;
+    }
+}

ErsatzTV.Tests/Application/Channels/AutoTuneNumberAllocatorTests.cs

@@ -0,0 +1,31 @@
+using System.Collections.Generic;
+using ErsatzTV.Application.Channels;
+using NUnit.Framework;
+using Shouldly;
+
+namespace ErsatzTV.Tests.Application.Channels;
+
+[TestFixture]
+public class AutoTuneNumberAllocatorTests
+{
+    [Test]
+    public void Allocate_Skips_Taken_Numbers()
+    {
+        var existing = new HashSet<string> { "500", "502" };
+        List<string> result = AutoTuneNumberAllocator.Allocate(500, 3, existing);
+        result.ShouldBe(new List<string> { "501", "503", "504" });
+    }
+
+    [Test]
+    public void Allocate_From_Empty_Is_Sequential()
+    {
+        List<string> result = AutoTuneNumberAllocator.Allocate(1, 3, new HashSet<string>());
+        result.ShouldBe(new List<string> { "1", "2", "3" });
+    }
+
+    [Test]
+    public void Allocate_Zero_Count_Is_Empty()
+    {
+        AutoTuneNumberAllocator.Allocate(500, 0, new HashSet<string>()).ShouldBeEmpty();
+    }
+}