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/mirror/assembler.md

@@ -0,0 +1,26 @@
+# Mirror Assembler Staffing & Milestones (Sprint 110.D)
+
+## Staffing
+- **Primary owner:** Alex Kim (Mirror Creator Guild)
+- **Backup:** Priya Desai (Exporter Guild)
+- **Observers:** AirGap Time Guild, CLI Guild
+
+## Milestones
+1. **M0 recap (done 2025-11-19):** thin bundle sample + layout note (`docs/modules/mirror/milestone-0-thin-bundle.md`).
+2. **M1 (due 2025-11-26):** implement deterministic assembler with manifest + CAS layout; DSSE signing profile `sovereign-default`; produce sample bundle tarball + hash.
+3. **M1.1 (due 2025-11-28):** TUF metadata generation (`root.json`, `snapshot.json`, `timestamp.json`) and mirror index manifest.
+4. **M2 (due 2025-12-05):** integrate time-anchor metadata (AirGap Time), export automation hooks (Export Center), and CLI verification path.
+
+## Responsibilities
+- Alex Kim: assembler core, DSSE signing, CAS layout.
+- Priya Desai: TUF metadata + exporter handoff.
+- AirGap Time Guild: time-anchor schema + verification guidance.
+- CLI Guild: `stella mirror verify` wiring and offline cache checks.
+
+## Artefact locations
+- Assembler design: `docs/modules/mirror/thin-bundle-assembler.md`
+- Staffing/plan (this doc): `docs/modules/mirror/assembler.md`
+- Sample bundles: `out/mirror/thin/mirror-thin-m0-sample.tar.gz` (future M1/M2 under `out/mirror/thin/`)
+
+## Risks
+- If M1 slips past 2025-11-26, Export Center and CLI tracks block; escalate to Exporter guild lead.

docs/modules/mirror/prep-56-001-thin-bundle.md

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+# Mirror Thin Bundle Prep — PREP-MIRROR-CRT-56-001 (Draft)
+
+Status: Draft (2025-11-20)
+Owners: Mirror Guild (Assembler)
+Scope: Capture requirements to start thin bundle v1 when upstream Sprint 110.D artefacts land.
+
+## Dependencies
+- Sprint 110.D assembler foundation (missing in repo).
+- Trust root list and TUF metadata locations from release pipeline.
+
+## Proposed thin bundle v1 shape
+- Container: tar.gz deterministic; root manifest `mirror.thin.manifest.json`.
+- Fields: `bundle_id`, `schema_version`=`mirror.thin.v1`, `created_at`, `source_registry`, `artifacts[] {digest, media_type, size}`, `trust_roots[]`, optional `attestations[]`.
+- Merkle root over files for audit.
+
+## Open decisions
+- Exact artifact set included in “thin” scope (SBOM only vs SBOM+metadata).
+- Required signatures (DSSE/Sigstore) and signer identities.
+- Retention/GC policy for thin bundles.
+
+## Handoff
+Use this as the PREP artefact for PREP-MIRROR-CRT-56-001; update when assembler foundation drops so schema can be finalized and aligned with `docs/modules/mirror/thin-bundle-assembler.md`.