fix(reforcexy): harden Optuna best-params persistence

fix(reforcexy): harden Optuna best-params persistence

Port the QuickAdapter Optuna best-params I/O hardening to ReforceXY's
save_best_trial_params / load_best_trial_params:

- Encode the complete pair identity in the filename via _sanitize_pair
  instead of the base currency only, so pairs sharing a base (e.g.
  BTC/USDT and BTC/USDC) no longer collide on one file.
- Warn and ignore a base-only legacy best-params file that shadows the
  pair-safe path.
- Write atomically (temp file + fsync + os.replace), preserving the
  previous file owner and mode.
- Serialize reads and writes with a flock lock file opened
  O_NOFOLLOW|O_NONBLOCK|O_CLOEXEC and guarded by an S_ISREG check.
- Fail closed when the live path is a symlink.
- Quarantine a corrupt best-params file on load and return None instead
  of raising, mirroring the existing journal quarantine.