Add tests for SBOM generation determinism across multiple formats

- Created `StellaOps.TestKit.Tests` project for unit tests related to determinism.
- Implemented `DeterminismManifestTests` to validate deterministic output for canonical bytes and strings, file read/write operations, and error handling for invalid schema versions.
- Added `SbomDeterminismTests` to ensure identical inputs produce consistent SBOMs across SPDX 3.0.1 and CycloneDX 1.6/1.7 formats, including parallel execution tests.
- Updated project references in `StellaOps.Integration.Determinism` to include the new determinism testing library.

src/AirGap/StellaOps.AirGap.Importer/Reconciliation/ArtifactIndex.cs

@@ -1,5 +1,7 @@
 namespace StellaOps.AirGap.Importer.Reconciliation;
 
+using StellaOps.Cryptography.Digests;
+
 /// <summary>
 /// Digest-keyed artifact index used by the evidence reconciliation flow.
 /// Designed for deterministic ordering and replay.
@@ -39,54 +41,7 @@ public sealed class ArtifactIndex
     public IEnumerable<KeyValuePair<string, ArtifactEntry>> GetAll() => _entries;
 
     public static string NormalizeDigest(string digest)
-    {
-        if (string.IsNullOrWhiteSpace(digest))
-        {
-            throw new ArgumentException("Digest is required.", nameof(digest));
-        }
-
-        digest = digest.Trim();
-
-        const string prefix = "sha256:";
-        string hex;
-
-        if (digest.StartsWith(prefix, StringComparison.OrdinalIgnoreCase))
-        {
-            hex = digest[prefix.Length..];
-        }
-        else if (digest.Contains(':', StringComparison.Ordinal))
-        {
-            throw new FormatException($"Unsupported digest algorithm in '{digest}'. Only sha256 is supported.");
-        }
-        else
-        {
-            hex = digest;
-        }
-
-        hex = hex.Trim().ToLowerInvariant();
-
-        if (hex.Length != 64 || !IsLowerHex(hex.AsSpan()))
-        {
-            throw new FormatException($"Invalid sha256 digest '{digest}'. Expected 64 hex characters.");
-        }
-
-        return prefix + hex;
-    }
-
-    private static bool IsLowerHex(ReadOnlySpan<char> value)
-    {
-        foreach (var c in value)
-        {
-            if ((c >= '0' && c <= '9') || (c >= 'a' && c <= 'f'))
-            {
-                continue;
-            }
-
-            return false;
-        }
-
-        return true;
-    }
+        => Sha256Digest.Normalize(digest, requirePrefix: false, parameterName: nameof(digest));
 }
 
 public sealed record ArtifactEntry(