docs(accounting): step-by-step tutorial for chains, accruals and gap tools

One walkthrough covering everything the three features do, aimed at the
accountant who has to run them and the developer who has to maintain
them. The two existing files stay as references; this is the guided path
through them.

21 Mermaid diagrams, all verified to parse:

  - what was broken, in three pictures
  - split vs move — why the allocation wizard could not express a chain
  - the settlement bug as a sequence, including why a balanced entry hid it
  - the accrue / collect / release cycle and where the double-count
    would have been
  - why a scanner beats event listeners, with the coach payroll listener
    as the worked example
  - the three-way match for procurement
  - first-run order, daily routine, monthly close
  - a troubleshooting decision tree and a table of the actual error
    messages
  - an ER diagram of the five new tables

Also states plainly the four things that still need a finance decision,
including the two that are deliberately left matching a mapping I
believe is wrong, because accrual and collection must agree.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>