Fix resume skipping an interrupted Hessian fit

[luciaquirke]

The step-2 resume check tests _step_complete(hessian_path), but the fit runs with run_path = {hessian_path}/{method} and promotes {hessian_path}/{method}.part -> {hessian_path}/{method}. Creating the .part directory also creates the parent hessian/, so after any interruption the parent exists, step_state reports complete, and a resumed run skips the unfinished fit — step 3 then crashes on missing factors (or, with a stale partial method dir, silently applies an incomplete Hessian).

Hit in practice when a shared volume filled mid-fit: every resumed pipeline jumped to apply_hessian and died with apply_hessian child exited with code 1; the only recovery was deleting hessian/ by hand and refitting from scratch.

Fix: check the path the fit actually promotes, {hessian_path}/{method} — the same path step 3 reads.

🤖 Generated with Claude Code

https://claude.ai/code/session_0167XKnE2yoCE162n3jHvxC1