Add new features and tests for AirGap and Time modules

- Introduced `SbomService` tasks documentation.
- Updated `StellaOps.sln` to include new projects: `StellaOps.AirGap.Time` and `StellaOps.AirGap.Importer`.
- Added unit tests for `BundleImportPlanner`, `DsseVerifier`, `ImportValidator`, and other components in the `StellaOps.AirGap.Importer.Tests` namespace.
- Implemented `InMemoryBundleRepositories` for testing bundle catalog and item repositories.
- Created `MerkleRootCalculator`, `RootRotationPolicy`, and `TufMetadataValidator` tests.
- Developed `StalenessCalculator` and `TimeAnchorLoader` tests in the `StellaOps.AirGap.Time.Tests` namespace.
- Added `fetch-sbomservice-deps.sh` script for offline dependency fetching.

src/AirGap/StellaOps.AirGap.Importer/Repositories/IBundleCatalogRepository.cs

@@ -0,0 +1,9 @@
+using StellaOps.AirGap.Importer.Models;
+
+namespace StellaOps.AirGap.Importer.Repositories;
+
+public interface IBundleCatalogRepository
+{
+    Task UpsertAsync(BundleCatalogEntry entry, CancellationToken cancellationToken);
+    Task<IReadOnlyList<BundleCatalogEntry>> ListAsync(string tenantId, CancellationToken cancellationToken);
+}

src/AirGap/StellaOps.AirGap.Importer/Repositories/IBundleItemRepository.cs

@@ -0,0 +1,9 @@
+using StellaOps.AirGap.Importer.Models;
+
+namespace StellaOps.AirGap.Importer.Repositories;
+
+public interface IBundleItemRepository
+{
+    Task UpsertManyAsync(IEnumerable<BundleItem> items, CancellationToken cancellationToken);
+    Task<IReadOnlyList<BundleItem>> ListByBundleAsync(string tenantId, string bundleId, CancellationToken cancellationToken);
+}

src/AirGap/StellaOps.AirGap.Importer/Repositories/InMemoryBundleRepositories.cs

@@ -0,0 +1,63 @@
+using StellaOps.AirGap.Importer.Models;
+
+namespace StellaOps.AirGap.Importer.Repositories;
+
+/// <summary>
+/// Deterministic in-memory implementations suitable for offline tests and as a template for Mongo-backed repos.
+/// Enforces tenant isolation and stable ordering (by BundleId then Path).
+/// </summary>
+public sealed class InMemoryBundleCatalogRepository : IBundleCatalogRepository
+{
+    private readonly Dictionary<string, List<BundleCatalogEntry>> _catalog = new(StringComparer.Ordinal);
+
+    public Task UpsertAsync(BundleCatalogEntry entry, CancellationToken cancellationToken)
+    {
+        cancellationToken.ThrowIfCancellationRequested();
+        var bucket = _catalog.GetValueOrDefault(entry.TenantId) ?? new List<BundleCatalogEntry>();
+        bucket.RemoveAll(e => e.BundleId == entry.BundleId);
+        bucket.Add(entry);
+        _catalog[entry.TenantId] = bucket;
+        return Task.CompletedTask;
+    }
+
+    public Task<IReadOnlyList<BundleCatalogEntry>> ListAsync(string tenantId, CancellationToken cancellationToken)
+    {
+        cancellationToken.ThrowIfCancellationRequested();
+        var items = _catalog.GetValueOrDefault(tenantId) ?? new List<BundleCatalogEntry>();
+        return Task.FromResult<IReadOnlyList<BundleCatalogEntry>>(items
+            .OrderBy(e => e.BundleId, StringComparer.Ordinal)
+            .ToList());
+    }
+}
+
+public sealed class InMemoryBundleItemRepository : IBundleItemRepository
+{
+    private readonly Dictionary<(string TenantId, string BundleId), List<BundleItem>> _items = new();
+
+    public Task UpsertManyAsync(IEnumerable<BundleItem> items, CancellationToken cancellationToken)
+    {
+        cancellationToken.ThrowIfCancellationRequested();
+        foreach (var item in items)
+        {
+            var key = (item.TenantId, item.BundleId);
+            if (!_items.TryGetValue(key, out var list))
+            {
+                list = new List<BundleItem>();
+                _items[key] = list;
+            }
+            list.RemoveAll(i => i.Path == item.Path);
+            list.Add(item);
+        }
+        return Task.CompletedTask;
+    }
+
+    public Task<IReadOnlyList<BundleItem>> ListByBundleAsync(string tenantId, string bundleId, CancellationToken cancellationToken)
+    {
+        cancellationToken.ThrowIfCancellationRequested();
+        var key = (tenantId, bundleId);
+        var list = _items.GetValueOrDefault(key) ?? new List<BundleItem>();
+        return Task.FromResult<IReadOnlyList<BundleItem>>(list
+            .OrderBy(i => i.Path, StringComparer.Ordinal)
+            .ToList());
+    }
+}