Bookkeeping export

src/export/ turns a received invoice into a balanced journal entry (buildJournalEntry(invoice, mapping)), and the entry into a SIE 4 file (type 4I, CP437, #VER/#TRANS), JSON or CSV.

• The caller supplies an AccountMapping: accounts payable, input VAT (one account or per rate), expense accounts by rule (line id, VAT category and rate, seller, item classification; first match wins, with a default) and one account each for reverse-charge VAT, withholding (per scheme), split-payment VAT, rounding, prepaid amounts, other taxes and KSeF settlement differences. --accounts mapping.json takes the same shape; without it the CLI and the site use BAS_EXAMPLE_MAPPING, an example onto the Swedish BAS chart (2440 payable, 2641 input VAT, 4010 / 5410 / 6xxx expenses) that is a demonstration and not advice on how to book anything.
• Accounts payable is credited with totals.payable. Credit notes reverse every posting through effectiveSign() (a credit note that keeps negative source amounts, like Finvoice INV02, is not reversed twice). Reverse charge (AE) books input and output VAT; the invoice states no VAT, so the rate comes from the invoice if it gives one and otherwise from reverseCharge.rate in the mapping, and a note records it. Corrections whose lines are before/after pairs (KSeF KOR) are booked per VAT category from the stated differences.
• The builder never invents a value and never emits an unbalanced entry: when the invoice’s own totals disagree with its lines, or an account the invoice needs is missing from the mapping, it throws a JournalError that names the amounts or the missing account. roundingTolerance books a stated small residual to the rounding account instead.
• SIE amounts are in the ledger currency (--currency, default SEK); an invoice in another currency is refused, no exchange rate is applied. Exports are byte-compared with test/export/expected/*.se in the tests.
• The demo site has a “Download journal entry” button that exports the shown invoice as SIE with the example mapping.