As a user whose crypto exchange (e.g. Kraken) exports each trade as two separate CSV rows (a spend row and a receive row) linked by a shared reference ID I want configure a `merge_column` in `config.yaml` (or via the TUI) that tells the preprocessor which column links rows of the same transaction, so that paired rows are merged into single atomic hledger transactions with correct cost notation so that Kraken-style split CSV exports produce the same unified journal entries as exchanges that use single-row atomic formats (e.g. Bitvavo), with proper `@@` total-cost notation, correct buy/sell direction for any currency pair (fiat-to-crypto, crypto-to-fiat, crypto-to-crypto, fiat-to-fiat), and explicit fee postings.