Compare prefixes ignoring case

src/Verify.Tests/Naming/PrefixUniqueTests.cs

@@ -0,0 +1,22 @@
+public class PrefixUniqueTests
+{
+    // The prefix maps to file names, and two prefixes differing only in case share
+    // one set of files on NTFS and APFS
+    [Fact]
+    public void CaseOnlyDifferenceIsNotUnique()
+    {
+        PrefixUnique.CheckPrefixIsUnique("PrefixUniqueTests.TheCase");
+
+        var exception = Assert.Throws<Exception>(
+            () => PrefixUnique.CheckPrefixIsUnique("prefixuniquetests.thecase"));
+
+        Assert.Contains("The prefix has already been used", exception.Message);
+    }
+
+    [Fact]
+    public void DistinctPrefixesAreUnique()
+    {
+        PrefixUnique.CheckPrefixIsUnique("PrefixUniqueTests.Distinct1");
+        PrefixUnique.CheckPrefixIsUnique("PrefixUniqueTests.Distinct2");
+    }
+}

src/Verify/Naming/PrefixUnique.cs

@@ -1,6 +1,9 @@
 static class PrefixUnique
 {
-    static ConcurrentDictionary<string, byte> prefixSet = [];
+    // Ignoring case, since the prefix maps to file names and NTFS and APFS are both
+    // case insensitive. Two prefixes differing only in case would silently share one
+    // set of files there, and a snapshot repository has to work on every platform.
+    static ConcurrentDictionary<string, byte> prefixSet = new(StringComparer.OrdinalIgnoreCase);
 
     public static void CheckPrefixIsUnique(string prefix)
     {

src/todo.md

@@ -79,7 +79,7 @@ All six resolved 2026-08-16 (five fixed here; the inline item resolved as by-des
 - [ ] **Mismatch crash for handle-based `FileStream` received streams.**
   `Verify/Compare/FileComparer.cs:50` — NotEqual fast path copies by `fileStream.Name` with no fallback; handle-based streams have `Name == "[Unknown]"`. First (New) run succeeds via the guarded `IoHelpers.WriteStream` path; later mismatches throw the generic "Failed to compare files".
 
-- [ ] **`PrefixUnique` set is case-sensitive on case-insensitive filesystems.**
+- [x] **`PrefixUnique` set is case-sensitive on case-insensitive filesystems.**
   `Verify/Naming/PrefixUnique.cs:3` — methods `Foo` and `foo` map to the same files on NTFS/APFS but pass the uniqueness check and silently clobber each other.
 
 - [ ] **`Counter` caches mix `Interlocked` counters with unsynchronized `Dictionary` writes.**