#1198 engine: document --spec and --spec-min-p in --help

说明

Both options are parsed (generate.cpp) and setup writes them into every run config ("--spec 4", "--spec-min-p 0.5"), and --calibrate measures --spec-min-p on the PC, but neither appears in the engine's --help: --mtp-max-t is listed and only mentions --spec in passing, so the two flags that decide how much the drafter proposes were the ones you could not look up.

Says what each one does, next to --mtp-max-t and --suffix-draft. No behaviour change.

Rebased onto the new `main` after the history cleanup. Replaces #1037.