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/modules/sbomservice/prep/2025-11-20-build-infra-prep.md

@@ -0,0 +1,15 @@
+# Build/Infra Prep — PREP-BUILD-INFRA-SBOM-SERVICE-GUILD-BLOCKED-M
+
+Status: Draft (2025-11-20)
+Owners: SBOM Service Guild
+Scope: Document restore/build blocking issues awaiting vetted feed/cache.
+
+## Blocker summary
+- Multiple restore attempts hang/fail; need vetted NuGet feed/cache for SBOM Service solution.
+
+## Needed actions
+- Provide approved offline feed snapshot for SBOM Service dependencies.
+- CI runner with feed configured to validate restore.
+
+## Handoff
+Use as PREP artefact; update once feed snapshot is supplied and restore passes.