feat: add Attestation Chain and Triage Evidence API clients and models

- Implemented Attestation Chain API client with methods for verifying, fetching, and managing attestation chains.
- Created models for Attestation Chain, including DSSE envelope structures and verification results.
- Developed Triage Evidence API client for fetching finding evidence, including methods for evidence retrieval by CVE and component.
- Added models for Triage Evidence, encapsulating evidence responses, entry points, boundary proofs, and VEX evidence.
- Introduced mock implementations for both API clients to facilitate testing and development.

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

@@ -108,4 +108,30 @@ public class RichGraphWriterTests
         Assert.Contains("\"type\":\"authRequired\"", json);
         Assert.Contains("\"guard_symbol\":\"sym:dotnet:B\"", json);
     }
+
+    [Fact]
+    public async Task UsesBlake3HashForDefaultProfile()
+    {
+        // WIT-013: Verify BLAKE3 is used for graph hashing
+        var writer = new RichGraphWriter(CryptoHashFactory.CreateDefault());
+        using var temp = new TempDir();
+
+        var union = new ReachabilityUnionGraph(
+            Nodes: new[]
+            {
+                new ReachabilityUnionNode("sym:dotnet:A", "dotnet", "method", "A")
+            },
+            Edges: Array.Empty<ReachabilityUnionEdge>());
+
+        var rich = RichGraphBuilder.FromUnion(union, "test-analyzer", "1.0.0");
+        var result = await writer.WriteAsync(rich, temp.Path, "analysis-blake3");
+
+        // Default profile (world) uses BLAKE3
+        Assert.StartsWith("blake3:", result.GraphHash);
+        Assert.Equal(64 + 7, result.GraphHash.Length); // "blake3:" (7) + 64 hex chars
+        
+        // Verify meta.json also contains the blake3-prefixed hash
+        var metaJson = await File.ReadAllTextAsync(result.MetaPath);
+        Assert.Contains("\"graph_hash\":\"blake3:", metaJson);
+    }
 }