docs: close M53 — the last two entries are wrong, not unresolved

[isayev]

No code change. The two M53 entries left open by #135 and #136 were checked against source, and neither claim holds.

ASE/thermo's model_name parameter — wrong

M53: "accepted, never read, yet calc_thermo:459 passes it explicitly."

Calculator stores it and passes it to to_model_species at thermo.py:536. It was genuinely unused when the audit was written; the C3/C4 species-conversion work made it live.

exceptions.py's "4 classes never raised" — wrong, and instructively so

Only ModelError is never raised directly, and that is deliberate:

class ModelLoadError(ModelError): ...
class NumericalError(ModelError): ...
# cli/errors.py:29
ModelError: 5,  # includes ModelLoadError / NumericalError

It exists as a base so both subclasses inherit exit code 5. Deleting it would break the exit-code scheme this release documents.

"Never raised" is not "unused" for a base class, and an audit that greps for raise X cannot tell the two apart. That is a false-positive shape worth remembering, not just a wrong entry.

Final tally for M53

verdict	count
already done before this effort	3
wrong on re-check	6
real, and removed in #135/#136	4

Of 13 entries, 4 were real. Nothing further to delete.

The lesson, recorded because it generalizes

A dead-code finding decays faster than any other kind. The same work that fixes defects revives symbols the audit recorded as dead — mol2atoms, STANDARD_PRESSURE, FailedMolecule, print_failures, and now model_name were all genuinely dead when written up and are all live now.

A mechanical sweep of this list would have deleted five live symbols, two of which vib_hessian depends on. That is the concrete argument for the re-verify-before-acting rule now written into the plan.

Both corrections are also written inline in the audit manifest, so neither can be re-derived from its finding ID by a future reader.