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src/__Libraries/StellaOps.VersionComparison/Comparers/StringVersionComparer.cs

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+// -----------------------------------------------------------------------------
+// StringVersionComparer.cs
+// Sprint: SPRINT_20251230_001_BE_backport_resolver (BP-101)
+// Task: Create fallback string version comparer
+// -----------------------------------------------------------------------------
+
+using System.Collections.Immutable;
+
+namespace StellaOps.VersionComparison.Comparers;
+
+/// <summary>
+/// Fallback version comparer that uses ordinal string comparison.
+/// Used when the package ecosystem is unknown or no specific comparator exists.
+/// </summary>
+public sealed class StringVersionComparer : IVersionComparator, IComparer<string>
+{
+    /// <summary>
+    /// Singleton instance.
+    /// </summary>
+    public static StringVersionComparer Instance { get; } = new();
+
+    private StringVersionComparer() { }
+
+    /// <inheritdoc />
+    public ComparatorType ComparatorType => ComparatorType.SemVer;
+
+    /// <inheritdoc />
+    public int Compare(string? x, string? y)
+    {
+        if (ReferenceEquals(x, y)) return 0;
+        if (x is null) return -1;
+        if (y is null) return 1;
+
+        return string.Compare(x, y, StringComparison.Ordinal);
+    }
+
+    /// <inheritdoc />
+    public VersionComparisonResult CompareWithProof(string? left, string? right)
+    {
+        var proofLines = new List<string>();
+
+        if (left is null && right is null)
+        {
+            proofLines.Add("Both versions are null: equal");
+            return new VersionComparisonResult(0, [.. proofLines], ComparatorType.SemVer);
+        }
+
+        if (left is null)
+        {
+            proofLines.Add("Left version is null: less than right");
+            return new VersionComparisonResult(-1, [.. proofLines], ComparatorType.SemVer);
+        }
+
+        if (right is null)
+        {
+            proofLines.Add("Right version is null: left is greater");
+            return new VersionComparisonResult(1, [.. proofLines], ComparatorType.SemVer);
+        }
+
+        var cmp = string.Compare(left, right, StringComparison.Ordinal);
+        var resultStr = cmp < 0 ? "left is older" : cmp > 0 ? "left is newer" : "equal";
+        var symbol = cmp < 0 ? "<" : cmp > 0 ? ">" : "==";
+
+        proofLines.Add($"String comparison (fallback): '{left}' {symbol} '{right}' ({resultStr})");
+
+        return new VersionComparisonResult(cmp, [.. proofLines], ComparatorType.SemVer);
+    }
+}