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.Importer.Tests/RootRotationPolicyTests.cs

@@ -0,0 +1,40 @@
+using StellaOps.AirGap.Importer.Validation;
+
+namespace StellaOps.AirGap.Importer.Tests;
+
+public class RootRotationPolicyTests
+{
+    [Fact]
+    public void RequiresTwoApprovers()
+    {
+        var policy = new RootRotationPolicy();
+        var result = policy.Validate(new Dictionary<string, byte[]>(), new Dictionary<string, byte[]> { ["k1"] = new byte[] { 1 } }, new[] { "a" });
+        Assert.False(result.IsValid);
+        Assert.Equal("rotation-dual-approval-required", result.Reason);
+    }
+
+    [Fact]
+    public void RejectsNoChange()
+    {
+        var policy = new RootRotationPolicy();
+        var result = policy.Validate(
+            new Dictionary<string, byte[]> { ["k1"] = new byte[] { 1 } },
+            new Dictionary<string, byte[]> { ["k1"] = new byte[] { 1 } },
+            new[] { "a", "b" });
+        Assert.False(result.IsValid);
+        Assert.Equal("rotation-no-change", result.Reason);
+    }
+
+    [Fact]
+    public void AcceptsRotationWithDualApproval()
+    {
+        var policy = new RootRotationPolicy();
+        var result = policy.Validate(
+            new Dictionary<string, byte[]> { ["old"] = new byte[] { 1 } },
+            new Dictionary<string, byte[]> { ["new"] = new byte[] { 2 } },
+            new[] { "a", "b" });
+
+        Assert.True(result.IsValid);
+        Assert.Equal("rotation-approved", result.Reason);
+    }
+}