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.

docs/samples/excititor/chunk-sample.ndjson

@@ -0,0 +1 @@
+{"chunk_id":"11111111-2222-3333-4444-555555555555","tenant":"acme","source":"ghsa","schema":"stellaops.vex.chunk.v1","items":[{"advisory_id":"GHSA-123","status":"affected","purl":"pkg:npm/foo@1.0.0"}],"provenance":{"fetched_at":"2025-11-20T00:00:00Z","artifact_sha":"abc"}}