Fix the legacy hardfork packaging fork-slot computation for Mesa preforks. `scripts/hardfork/create_runtime_config.sh` recomputed `proof.fork.global_slot_since_genesis` from the wall-clock genesis window using a hard-coded 180 s/slot (the Berkeley slot time); Mesa runs at 90 s/slot, so a `mesa -> mesa` fork came out off by 2x (e.g. `456780` instead of `458220`). The fork slot is now taken verbatim from the fork config's `proof.fork.global_slot_since_genesis` and the wall-clock-to-slots recompute is removed. Adds a regression test (`create-runtime-config-tests.sh`, run by `AutoHardforkTest`). Fixes #18980.
