docs: step-by-step accounting guide for the finance review

Terminology, screen-by-screen walkthrough, a demo running order, and — most
importantly — an honest status matrix: 267 money paths mapped across 67 modules,
71 of which actually reach the ledger.

Section 6 lists what must NOT be demoed or claimed. Being caught overstating in
front of accountants is far worse than a known, quantified gap, so the guide
leads with the exclusions and gives the exact wording for the hard question.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>