feat: Add in-memory implementations for issuer audit, key, repository, and trust management

- Introduced InMemoryIssuerAuditSink to retain audit entries for testing.
- Implemented InMemoryIssuerKeyRepository for deterministic key storage.
- Created InMemoryIssuerRepository to manage issuer records in memory.
- Added InMemoryIssuerTrustRepository for managing issuer trust overrides.
- Each repository utilizes concurrent collections for thread-safe operations.
- Enhanced deprecation tracking with a comprehensive YAML schema for API governance.

src/Scanner/__Tests/StellaOps.Scanner.Reachability.Tests/RichGraphWriterTests.cs

@@ -1,5 +1,6 @@
 using System.IO;
 using System.Threading.Tasks;
+using StellaOps.Cryptography;
 using StellaOps.Scanner.Reachability;
 using Xunit;
 
@@ -10,7 +11,7 @@ public class RichGraphWriterTests
     [Fact]
     public async Task WritesCanonicalGraphAndMeta()
     {
-        var writer = new RichGraphWriter();
+        var writer = new RichGraphWriter(CryptoHashFactory.CreateDefault());
         using var temp = new TempDir();
 
         var union = new ReachabilityUnionGraph(
@@ -31,7 +32,7 @@ public class RichGraphWriterTests
         Assert.True(File.Exists(result.MetaPath));
         var json = await File.ReadAllTextAsync(result.GraphPath);
         Assert.Contains("richgraph-v1", json);
-        Assert.StartsWith("sha256:", result.GraphHash);
+        Assert.Contains(":", result.GraphHash); // hash format: algorithm:digest
         Assert.Equal(2, result.NodeCount);
         Assert.Equal(1, result.EdgeCount);
     }