Add durable recipe coordinator and trusted publication path

[dovvnloading]

Summary

This PR implements the next planned Phase 2 slice: a durable, qualification-only recipe coordinator and trusted artifact-publication path for the fixed-function image transform.

What changed

• Added RecipeImageRequest, RecipeWorkerAttempt, RecipeWorkerOutput, and stable RecipeExecutionError contracts.
• Added RecipeWorkerClient for the existing authenticated broker/worker protocol, including bounded chunking, response identity checks, digest/MIME/format validation, cancellation, timeout, and transport failure handling.
• Added RecipeExecutionCoordinator with SQLite-backed idempotency, per-job leases, startup recovery, owner-scoped artifact reads, cancellation, private staging, and all-or-nothing publication through ArtifactBoundary.collect_outputs.
• Added repository helpers for owner-scoped artifact metadata and guarded queued payload replacement.
• Added adversarial coordinator/worker tests and a dedicated ADR; aligned the Phase 2 evidence and related ADR next gates.

Safety and scope

• Durable payloads contain opaque artifact IDs and canonical plan data only; no source paths, shell/command fields, model authority, or executable paths are accepted.
• Worker attempts are injected and must already be bound to the existing qualification/native broker gates. There is no host-process or in-process fallback.
• Outputs are published only after exact-claim validation, byte-derived MIME checks, size/hash checks, and cancellation-race cleanup.
• This PR does not add a public API route or make the normal application default-on. UI/API exposure and qualified attempt-factory wiring are the next planned stage.
• External review, production signing, and trusted release roots remain optional official-release hardening and are not required for this open-source slice.

Validation

• C:\Python314\python.exe -m pytest -q ? 325 passed, 1 skipped (Windows-inapplicable native test)
• C:\Python314\python.exe -m ruff check backend/cortex_backend/execution/__init__.py backend/cortex_backend/execution/repository.py backend/cortex_backend/execution/recipe_coordinator.py tests/test_phase2_recipe_coordinator.py ? passed
• git diff --cached --check ? passed

The unrelated working-tree edit in Cortex.pyproj was intentionally excluded.