Add a `--identity-conversion` flag to `runtime_genesis_ledger` for Mesa → Mesa hardfork packaging. By default the tool still applies the Berkeley → Mesa vesting conversion (`Account.slot_reduction_update`) when a hardfork slot is derived from `--prefork-genesis-config`. With `--identity-conversion` the accounts are loaded verbatim (the conversion is the identity, matching the daemon's `migrate_to_mesa`) and `--prefork-genesis-config` is ignored, so a Mesa-sourced fork no longer re-bases already-Mesa timed accounts into a divergent genesis-ledger root. Fixes #18981.
