Fix compute-control artifact reuse

[TacoRocket]

What changed

• register workloads and managed-identities in the grouped-source reuse model map
• add empty grouped-source fallback shapes for those two compute-control backing commands
• add focused regressions for cached compute-control reuse, --live-only, and stale-artifact fallback

Why

Cached chains compute-control reruns were failing before grouped output finalization because the strict artifact-reuse path did not know how to validate or synthesize empty outputs for workloads and managed-identities. Live-only runs still worked, which made this a narrow grouped reuse seam.

Impact

• cached chains compute-control now reuses matching artifacts cleanly
• chains compute-control --live-only still stays fully live
• stale compute-control source artifacts still fall back live and surface honest provenance

Validation

• PYTHONPATH=src python3 -m pytest -q tests/test_chain_scaffold.py -k "compute_control_sources_support_reuse_and_empty_fallbacks" tests/test_cli_smoke.py -k "compute_control_reuses_matching_source_artifacts_by_default or compute_control_live_only_bypasses_cached_artifacts or compute_control_falls_back_live_on_artifact_mismatch or chains_compute_control_json"
• python3 -m ruff check src/azurefox/chains/registry.py tests/test_chain_scaffold.py tests/test_cli_smoke.py
• pre-push guardrail: 429 passed, 2 deselected