Add docs and unit tests for `IList.LowerBoundBinarySearch` extension

without d29da10e9, the final case in this test method would throw an exception

src/BizHawk.Common/Extensions/CollectionExtensions.cs

@@ -17,6 +17,14 @@ namespace BizHawk.Common.CollectionExtensions
 			return desc ? source.OrderByDescending(keySelector) : source.OrderBy(keySelector);
 		}
 
+		/// <summary>Implements an indirected binary search.</summary>
+		/// <return>
+		/// The index of the element whose key matches <paramref name="key"/>;
+		/// or if none match, the index of the element whose key is closest and lower;
+		/// or if all elements' keys are higher, <c>-1</c>.<br/>
+		/// (Equivalently: If none match, 1 less than the index where inserting an element with the given <paramref name="key"/> would keep the list sorted)
+		/// </return>
+		/// <remarks>The returned index may not be accurate if <paramref name="list"/> is not sorted in ascending order with respect to <paramref name="keySelector"/>.</remarks>
 		public static int LowerBoundBinarySearch<T, TKey>(this IList<T> list, Func<T, TKey> keySelector, TKey key)
 			where TKey : IComparable<TKey>
 		{

src/BizHawk.Tests/Common/CollectionExtensions/CollectionExtensionTests.cs

@@ -75,6 +75,18 @@ namespace BizHawk.Tests.Common.CollectionExtensions
 			Assert.AreEqual(0, Array.Empty<int>().ConcatArray(Array.Empty<int>()).Length);
 		}
 
+		[TestMethod]
+		public void TestLowerBoundBinarySearch()
+		{
+			List<string> a = new(new[] { "a", "abc", "abcde", "abcdef", "abcdefg" });
+			Assert.AreEqual(-1, a.LowerBoundBinarySearch(static s => s.Length, 0), "length 0");
+			Assert.AreEqual(0, a.LowerBoundBinarySearch(static s => s.Length, 1), "length 1");
+			Assert.AreEqual(1, a.LowerBoundBinarySearch(static s => s.Length, 4), "length 4");
+			Assert.AreEqual(2, a.LowerBoundBinarySearch(static s => s.Length, 5), "length 5");
+			Assert.AreEqual(4, a.LowerBoundBinarySearch(static s => s.Length, 7), "length 7");
+			Assert.AreEqual(4, a.LowerBoundBinarySearch(static s => s.Length, 8), "length 8");
+		}
+
 		[TestMethod]
 		public void TestRemoveAll()
 		{