Add Canonical JSON serialization library with tests and documentation

- Implemented CanonJson class for deterministic JSON serialization and hashing.
- Added unit tests for CanonJson functionality, covering various scenarios including key sorting, handling of nested objects, arrays, and special characters.
- Created project files for the Canonical JSON library and its tests, including necessary package references.
- Added README.md for library usage and API reference.
- Introduced RabbitMqIntegrationFactAttribute for conditional RabbitMQ integration tests.

src/Signer/StellaOps.Signer/StellaOps.Signer.Core/PredicateTypes.cs

@@ -74,6 +74,22 @@ public static class PredicateTypes
     /// </summary>
     public const string StellaOpsReachabilityWitness = "stella.ops/reachabilityWitness@v1";
 
+    /// <summary>
+    /// StellaOps Path Witness predicate type for DSSE attestations.
+    /// Sprint: SPRINT_3700_0001_0001 (WIT-007C)
+    /// Cryptographic proof of a specific entrypoint → sink path.
+    /// Used by PathWitnessBuilder to sign individual path witnesses.
+    /// </summary>
+    public const string StellaOpsPathWitness = "stella.ops/pathWitness@v1";
+
+    /// <summary>
+    /// StellaOps Reachability Drift predicate type for DSSE attestations.
+    /// Sprint: SPRINT_3600_0004_0001_ui_evidence_chain (UI-014)
+    /// Cryptographic proof of reachability changes between scans.
+    /// Used by DriftAttestationService to sign drift analysis results.
+    /// </summary>
+    public const string StellaOpsReachabilityDrift = "stellaops.dev/predicates/reachability-drift@v1";
+
     /// <summary>
     /// CycloneDX SBOM predicate type.
     /// </summary>
@@ -123,7 +139,9 @@ public static class PredicateTypes
         return predicateType == StellaOpsGraph
             || predicateType == StellaOpsReplay
             || predicateType == StellaOpsEvidence
-            || predicateType == StellaOpsReachabilityWitness;
+            || predicateType == StellaOpsReachabilityWitness
+            || predicateType == StellaOpsPathWitness
+            || predicateType == StellaOpsReachabilityDrift;
     }
 
     /// <summary>
@@ -147,6 +165,8 @@ public static class PredicateTypes
             StellaOpsVexDecision,
             StellaOpsGraph,
             StellaOpsReachabilityWitness,
+            StellaOpsPathWitness,
+            StellaOpsReachabilityDrift,
             // Third-party types
             CycloneDxSbom,
             SpdxSbom,