Add receipt input JSON and SHA256 hash for CVSS policy scoring tests

- Introduced a new JSON fixture `receipt-input.json` containing base, environmental, and threat metrics for CVSS scoring.
- Added corresponding SHA256 hash file `receipt-input.sha256` to ensure integrity of the JSON fixture.

src/Concelier/__Libraries/StellaOps.Concelier.Storage.Postgres/Converters/Importers/ParityRunner.cs

@@ -0,0 +1,38 @@
+using System.Collections.Immutable;
+using StellaOps.Concelier.Storage.Postgres.Repositories;
+
+namespace StellaOps.Concelier.Storage.Postgres.Converters.Importers;
+
+/// <summary>
+/// Compares imported advisory snapshots between sources to ensure parity before cutover.
+/// </summary>
+public sealed class ParityRunner
+{
+    private readonly IAdvisorySnapshotRepository _snapshots;
+
+    public ParityRunner(IAdvisorySnapshotRepository snapshots)
+    {
+        _snapshots = snapshots;
+    }
+
+    /// <summary>
+    /// Compares two feed snapshots by advisory keys; returns true when keys match exactly.
+    /// </summary>
+    public async Task<ParityResult> CompareAsync(Guid feedSnapshotA, Guid feedSnapshotB, CancellationToken cancellationToken = default)
+    {
+        var a = await _snapshots.GetByFeedSnapshotAsync(feedSnapshotA, cancellationToken).ConfigureAwait(false);
+        var b = await _snapshots.GetByFeedSnapshotAsync(feedSnapshotB, cancellationToken).ConfigureAwait(false);
+
+        var setA = a.Select(s => s.AdvisoryKey).ToImmutableSortedSet(StringComparer.OrdinalIgnoreCase);
+        var setB = b.Select(s => s.AdvisoryKey).ToImmutableSortedSet(StringComparer.OrdinalIgnoreCase);
+
+        var missingInB = setA.Except(setB).ToArray();
+        var missingInA = setB.Except(setA).ToArray();
+
+        var match = missingInA.Length == 0 && missingInB.Length == 0;
+
+        return new ParityResult(match, missingInA, missingInB);
+    }
+
+    public sealed record ParityResult(bool Match, IReadOnlyList<string> MissingInA, IReadOnlyList<string> MissingInB);
+}