LU2-WP06 - Execution contract, recovery and diagnostics

WP06 installs a small additive contract around existing routes. It does not replace the router and does not alter FEM assembly, solver tolerances or formulations.

Lifecycle

ExecutionSession enforces the following fail-closed lifecycle:

CREATED -> VALIDATED -> RUNNING -> CONVERGED -> CHECKPOINTED -> RECOVERED -> RUNNING

Any terminal failure is represented by FAILED and requires a structured diagnostic. Invalid transitions are rejected. Each session records the source SHA, route, backend, solver status, provenance, evidence link and state history.

Diagnostics

The stable taxonomy is defined in src/solveur/execution/diagnostics.py. Codes use the QF-EXEC- namespace and carry a category, deterministic message, minimal context and a recoverability flag. It covers model/input errors, unsupported capabilities, unavailable backends, non-convergence, resource limitation, numerical invalidity, checkpoint corruption, recovery failure and unclassified execution failure.

Existing solver exceptions can be mapped with diagnostic_from_exception. Unknown exceptions remain EXECUTION_FAILURE; they cannot become a PASS.

Recovery boundary

Existing versioned NPZ checkpoint stores remain the persistence authority. Targeted tests demonstrate valid round trips and reject corrupted or incompatible checkpoints. The recoverable boundary is limited to fixed-load nonlinear static, arc-length nonlinear static and Newmark transient routes. Linear static, buckling, harmonic, geometric nonlinear and large distributed routes have no restart claim under this WP06 contract.

This scope is intentionally bounded: a valid checkpoint does not imply a universal fault-tolerance guarantee, and an unavailable or invalid recovery never becomes a successful result.

Evidence and tests

The machine-readable contract is qualification/0_2_7/wp06_execution_contract.json, with closeout state in qualification/0_2_7/lu2_wp06_state.json. The new focused tests are tests/unit/test_execution_contract.py; legacy nonlinear and dynamic checkpoint tests remain unchanged and continue to cover their existing paths.

WP06 is PASS_WITH_LIMITATIONS. No heavy benchmark, full regression, new physics or numerical formulation change was performed.