Add fixed recipe worker protocol boundary

[dovvnloading]

Summary

• add the transport-neutral recipe.worker.v1 contract with bounded prepare, input chunking, completion, cancellation, and output collection messages
• add a one-request worker state machine with per-chunk and whole-stream hash validation, cancellation race handling, and redacted output metadata
• add a reproducible PyInstaller worker package definition and Windows build script
• document that the package exits with code 78 until the native broker loop is implemented; update the Phase 2 evidence log and ADR

Validation

• python -m pytest -q — 226 passed, 1 skipped
• python -m ruff check backend/cortex_backend/execution/worker_protocol.py tests/test_phase2_worker_protocol.py
• python -m compileall -q backend/cortex_backend/execution packaging/recipe_worker tests/test_phase2_worker_protocol.py
• python tools/generate_contracts.py
• git diff --check
• PyInstaller build produced dist/recipe-runtime/recipe_worker.exe; direct launch returned exit code 78

Gate status

This PR does not authorize provider execution. The worker package is unsigned and launch-refusing. The next stage must implement the reviewed native broker loop, launch the verified worker suspended under AppContainer/LPAC and Job Object policy, bind the live broker session to the actual worker PID/AppContainer token, and run the hostile decoder/cancellation corpus before lifecycle enablement.