[Orders] Add support for sequential/ incremental invoice number in purchase receipts

## Intro

Some countries (EU, India, etc) have a legal requirement of a sequentially incremental invoice/purchase receipt for every incoming transaction.

## Proposal

Add a serial number merge code to purchase receipts. Based on user feedback, I have created a gist that does this in a slightly hacky way in the meantime: https://gist.github.com/actual-saurabh/a479571b6ceb5e6b32e30f31575050f0

It ignores test mode transactions and all failed transactions to generate incremental serial numbers. It includes refunded transactions (as per legal requirement, the serial number stays as it is even if the transaction was refunded subsequently and will be used for reference).

The gist contains a way to set a length with leading zeroes (as is preferred/required in some places) and even add a prefix.

It is hacky and the first run (where existing transactions get assigned a serial number) needs a full review but I wanted to note this down to be able to track and continue discussing.

Start by reviewing the linked gist and the purchase-receipt merge-code flow. Confirm how successful, failed, test-mode, and refunded transactions should affect sequential numbering, including the first run for existing transactions. Done means the supported serial number behavior, prefixes, and leading-zero formatting are defined and covered by the project’s relevant tests.

Written by the indexing model from the issue text.