feat: Add VEX Status Chip component and integration tests for reachability drift detection

- Introduced `VexStatusChipComponent` to display VEX status with color coding and tooltips.
- Implemented integration tests for reachability drift detection, covering various scenarios including drift detection, determinism, and error handling.
- Enhanced `ScannerToSignalsReachabilityTests` with a null implementation of `ICallGraphSyncService` for better test isolation.
- Updated project references to include the new Reachability Drift library.

tests/reachability/StellaOps.ScannerSignals.IntegrationTests/ScannerToSignalsReachabilityTests.cs

@@ -69,6 +69,7 @@ public sealed class ScannerToSignalsReachabilityTests
             callgraphRepo,
             reachabilityStore,
             new CallgraphNormalizationService(),
+            new NullCallGraphSyncService(),
             Options.Create(new SignalsOptions()),
             TimeProvider.System,
             NullLogger<CallgraphIngestionService>.Instance);
@@ -205,6 +206,21 @@ public sealed class ScannerToSignalsReachabilityTests
             storage[document.SubjectKey] = document;
             return Task.FromResult(document);
         }
+
+        public Task<IReadOnlyList<ReachabilityFactDocument>> GetExpiredAsync(DateTimeOffset cutoff, int limit, CancellationToken cancellationToken)
+            => Task.FromResult((IReadOnlyList<ReachabilityFactDocument>)Array.Empty<ReachabilityFactDocument>());
+
+        public Task<bool> DeleteAsync(string subjectKey, CancellationToken cancellationToken)
+        {
+            var removed = storage.Remove(subjectKey);
+            return Task.FromResult(removed);
+        }
+
+        public Task<int> GetRuntimeFactsCountAsync(string subjectKey, CancellationToken cancellationToken)
+            => Task.FromResult(0);
+
+        public Task TrimRuntimeFactsAsync(string subjectKey, int maxCount, CancellationToken cancellationToken)
+            => Task.CompletedTask;
     }
 
     private sealed class InMemoryReachabilityCache : IReachabilityCache
@@ -240,6 +256,28 @@ public sealed class ScannerToSignalsReachabilityTests
 
         public Task<int> CountBySubjectAsync(string subjectKey, CancellationToken cancellationToken) =>
             Task.FromResult(0);
+
+        public Task BulkUpdateAsync(IEnumerable<UnknownSymbolDocument> items, CancellationToken cancellationToken) =>
+            Task.CompletedTask;
+
+        public Task<IReadOnlyList<string>> GetAllSubjectKeysAsync(CancellationToken cancellationToken) =>
+            Task.FromResult((IReadOnlyList<string>)Array.Empty<string>());
+
+        public Task<IReadOnlyList<UnknownSymbolDocument>> GetDueForRescanAsync(
+            UnknownsBand band,
+            int limit,
+            CancellationToken cancellationToken) =>
+            Task.FromResult((IReadOnlyList<UnknownSymbolDocument>)Array.Empty<UnknownSymbolDocument>());
+
+        public Task<IReadOnlyList<UnknownSymbolDocument>> QueryAsync(
+            UnknownsBand? band,
+            int limit,
+            int offset,
+            CancellationToken cancellationToken) =>
+            Task.FromResult((IReadOnlyList<UnknownSymbolDocument>)Array.Empty<UnknownSymbolDocument>());
+
+        public Task<UnknownSymbolDocument?> GetByIdAsync(string id, CancellationToken cancellationToken) =>
+            Task.FromResult<UnknownSymbolDocument?>(null);
     }
 
     private sealed class NullEventsPublisher : IEventsPublisher
@@ -247,6 +285,15 @@ public sealed class ScannerToSignalsReachabilityTests
         public Task PublishFactUpdatedAsync(ReachabilityFactDocument fact, CancellationToken cancellationToken) => Task.CompletedTask;
     }
 
+    private sealed class NullCallGraphSyncService : ICallGraphSyncService
+    {
+        public Task<CallGraphSyncResult> SyncAsync(Guid scanId, string artifactDigest, CallgraphDocument document, CancellationToken cancellationToken)
+            => Task.FromResult(new CallGraphSyncResult(scanId, 0, 0, 0, false, 0L));
+
+        public Task DeleteByScanAsync(Guid scanId, CancellationToken cancellationToken)
+            => Task.CompletedTask;
+    }
+
     private sealed class InMemoryCallgraphArtifactStore : ICallgraphArtifactStore
     {
         private readonly Dictionary<string, byte[]> artifacts = new(StringComparer.OrdinalIgnoreCase);