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/__Libraries/StellaOps.Cryptography/DefaultCryptoHmac.cs

@@ -61,7 +61,7 @@ public sealed class DefaultCryptoHmac : ICryptoHmac
     }
 
     public string ComputeHmacHexForPurpose(ReadOnlySpan<byte> key, ReadOnlySpan<byte> data, string purpose)
-        => Convert.ToHexString(ComputeHmacForPurpose(key, data, purpose)).ToLowerInvariant();
+        => Convert.ToHexStringLower(ComputeHmacForPurpose(key, data, purpose));
 
     public string ComputeHmacBase64ForPurpose(ReadOnlySpan<byte> key, ReadOnlySpan<byte> data, string purpose)
         => Convert.ToBase64String(ComputeHmacForPurpose(key, data, purpose));
@@ -78,7 +78,7 @@ public sealed class DefaultCryptoHmac : ICryptoHmac
     public async ValueTask<string> ComputeHmacHexForPurposeAsync(ReadOnlyMemory<byte> key, Stream stream, string purpose, CancellationToken cancellationToken = default)
     {
         var bytes = await ComputeHmacForPurposeAsync(key, stream, purpose, cancellationToken).ConfigureAwait(false);
-        return Convert.ToHexString(bytes).ToLowerInvariant();
+        return Convert.ToHexStringLower(bytes);
     }
 
     #endregion