Credit notes calculations and validations

Shares all invoice validations (issue-date window, items, allowances/charges, delivery, transaction types) — differences only:

• credit_note_reason_code: valid code if provided.
• original_invoice_id: required unless reason is Volume Discount.
• Type mapping to original invoice: 380→381, 480→81, 389→261. Type 81 carries the same E/O/Z + company_id restriction as 480. Type 261 (self-billed credit note) needs the branch company/postal check — same bypass gap as self-billed invoices above.

When original_invoice_id is provided:

• Must reference an invoice, not another credit note.
• customer_party_id (or supplier_id for self-billed) must match the original.
• issue_date ≥ original's issue_date.
• document_currency_code and tax_currency_exchange_rate (if provided) must match the original.
• transaction_types, if provided, must match the original; otherwise inherited.
• Every credit-note item must reference an item on the original invoice.
• Per-item line amount can't exceed the original line; cumulative credited amount across all prior credit notes for that item can't exceed the original line amount.
• Total payable_amount can't exceed the original invoice's.