feat: Implement DefaultCryptoHmac for compliance-aware HMAC operations

- Added DefaultCryptoHmac class implementing ICryptoHmac interface.
- Introduced purpose-based HMAC computation methods.
- Implemented verification methods for HMACs with constant-time comparison.
- Created HmacAlgorithms and HmacPurpose classes for well-known identifiers.
- Added compliance profile support for HMAC algorithms.
- Included asynchronous methods for HMAC computation from streams.

src/Web/StellaOps.Web/TASKS.md

@@ -19,6 +19,7 @@
 | UI-POLICY-23-001 | DONE (2025-12-05) | Workspace route `/policy-studio/packs` with pack list + quick actions; cached pack store with offline fallback. |
 | UI-POLICY-23-002 | DONE (2025-12-05) | YAML editor route `/policy-studio/packs/:packId/yaml` with canonical preview and lint diagnostics. |
 | UI-POLICY-23-003 | DONE (2025-12-05) | Rule Builder route `/policy-studio/packs/:packId/rules` with guided inputs and deterministic preview JSON. |
+| UI-POLICY-23-004 | DONE (2025-12-05) | Approval workflow UI updated with readiness checklist, schedule window card, comment thread, and two-person indicator; tests attempted but Angular CLI hit missing rxjs util module. |
 | UI-POLICY-23-005 | DONE (2025-12-05) | Simulator updated with SBOM/advisory pickers and explain trace view; uses PolicyApiService simulate. |
 | UI-POLICY-23-006 | DOING (2025-12-05) | Explain view route `/policy-studio/packs/:packId/explain/:runId` with trace + JSON export; PDF export pending backend. |
 | UI-POLICY-23-001 | DONE (2025-12-05) | Workspace route `/policy-studio/packs` with pack list + quick actions; cached pack store with offline fallback. |