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.

tests/AirGap/StellaOps.AirGap.Time.Tests/StalenessCalculatorTests.cs

@@ -0,0 +1,43 @@
+using StellaOps.AirGap.Time.Models;
+using StellaOps.AirGap.Time.Services;
+
+namespace StellaOps.AirGap.Time.Tests;
+
+public class StalenessCalculatorTests
+{
+    [Fact]
+    public void UnknownWhenNoAnchor()
+    {
+        var calc = new StalenessCalculator();
+        var result = calc.Evaluate(TimeAnchor.Unknown, StalenessBudget.Default, DateTimeOffset.UnixEpoch);
+        Assert.False(result.IsWarning);
+        Assert.False(result.IsBreach);
+    }
+
+    [Fact]
+    public void BreachWhenBeyondBudget()
+    {
+        var anchor = new TimeAnchor(DateTimeOffset.UnixEpoch, "source", "fmt", "fp", "digest");
+        var budget = new StalenessBudget(10, 20);
+        var calc = new StalenessCalculator();
+
+        var result = calc.Evaluate(anchor, budget, DateTimeOffset.UnixEpoch.AddSeconds(25));
+
+        Assert.True(result.IsBreach);
+        Assert.True(result.IsWarning);
+        Assert.Equal(25, result.AgeSeconds);
+    }
+
+    [Fact]
+    public void WarningWhenBetweenWarningAndBreach()
+    {
+        var anchor = new TimeAnchor(DateTimeOffset.UnixEpoch, "source", "fmt", "fp", "digest");
+        var budget = new StalenessBudget(10, 20);
+        var calc = new StalenessCalculator();
+
+        var result = calc.Evaluate(anchor, budget, DateTimeOffset.UnixEpoch.AddSeconds(15));
+
+        Assert.True(result.IsWarning);
+        Assert.False(result.IsBreach);
+    }
+}