Sales Tax Presets and Line Calculation
======================================

Preset behavior
---------------

``SalesTaxPreset`` stores jurisdiction defaults by country and region.

Seeded defaults include all Canadian provinces and territories:

* AB, BC, MB, NB, NL, NS, NT, NU, ON, PE, QC, SK, YT

Preset resolution order
-----------------------

1. If Tax Mode = ``Manual``, manual rates are used.
2. If Tax Mode = ``Auto`` and preset is selected on the invoice, preset rates are used.
3. If Tax Mode = ``Auto`` and no preset is selected, the system searches active
   presets by ``ship_to_country_code`` + ``ship_to_region_code``.
4. If no preset matches, fallback defaults are used (for Canada/US only).

Line-level (Xero-style) calculation
-----------------------------------

Tax is computed per line and rounded per line item to two decimals, then summed.
This matches common accounting behavior where line-level rounding may differ from
single-document rounding.

Algorithm summary:

1. Build invoice lines from ``ItemTransactionModel`` plus optional Shipping line.
2. Allocate invoice-level discount proportionally across taxable lines.
3. For each line:
   * ``taxable_line_amount = line_amount - allocated_discount``
   * ``line_tax = round(taxable_line_amount * tax_rate, 2)``
4. Sum all line tax components for totals.

Tax-exempt items
----------------

Any item with ``item_model.additional_info["sales_tax_exempt"] = true`` is
treated as non-taxable in this flow.