archive-hardfork-toolbox: add a `populate-genesis-accounts` subcommand to repair a hardfork archive whose fork genesis block is missing its ledger accounts. Fixes #19058.

After a hard fork the chain restarts at a fork genesis block whose `blockchain_length` is `fork.blockchain_length + 1`. Only `add_genesis_accounts` writes the genesis ledger into `accounts_accessed`, and the archive runs it at startup only when it is (re)started with the fork `--config-file`. An archive that ingested the fork blocks without that step — restored from a dump, or started without the config — ends up with no `accounts_accessed` rows at the fork genesis block, so Rosetta reports zero balances there and `rosetta-cli check:data` fails at the first post-fork block.

`mina-archive-hardfork-toolbox populate-genesis-accounts --postgres-uri <uri> --config-file <runtime-config>` runs `add_genesis_accounts` against an existing archive, backfilling the (fork) genesis ledger onto the already-present fork genesis block, so operators can repair an affected archive without a full re-ingest. `--chunks-length` (default 100) bounds how many accounts are inserted per transaction.

Also adds a regression test in `src/app/archive/lib/test.ml` that ingests a fork genesis block with no accounts, asserts `accounts_accessed = 0`, then runs `add_genesis_accounts` and asserts every ledger account is attached.
