fix(quickadapter): prevent best-params lock hang on FIFO paths (#191)

fix(quickadapter): prevent best-params lock hang on FIFO paths (#191)

Open the best-params lock with O_NONBLOCK so a pre-existing FIFO at
.optuna-best-params.lock no longer blocks the O_RDONLY shared-load open
until a writer appears (O_NOFOLLOW rejects symlinks but not FIFOs),
hanging strategy startup before the S_ISREG guard can reject the
non-regular file. The flag is inert on regular files and does not affect
the subsequent blocking flock() (governed by LOCK_NB, not the fd flag).

Addresses the unresolved P2 review thread on merged PR #185.