fix(quickadapter): make best-params persistence atomic (#185)

fix(quickadapter): make best-params persistence atomic (#185)

* fix(quickadapter): make best-params persistence atomic

* fix(quickadapter): preserve best-params startup contract

* fix(quickadapter): preserve best-params access contracts

* fix(quickadapter): allow shared best-params reads

* refactor(quickadapter): harmonize best-params quarantine terminology

Align the Optuna best-params quarantine with the established journal
quarantine sibling (_optuna_quarantine_journal): source the artifact tag
from a new _OPTUNA_BEST_PARAMS_QUARANTINE_TAG constant and use "corrupt"
consistently in the docstring and log message (the function name and
.corrupt- artifact suffix already used "corrupt"; only the docstring and
warning diverged as "malformed"). Add a docstring to
_reject_optuna_best_params_symlink for parity with its sibling helpers.

On-disk artifacts and load results are byte-identical; only the log
wording and tag source change.