docs(accounting): full Egyptian-Arabic guide with diagrams

One document covering the whole revenue posting system end to end: how a
payment becomes a journal entry, the 97 streams across 14 categories, the
six stages and which apply to which category, direction, the five scopes,
the wizard step by step, the allocation waterfall, line types, VAT,
deferred revenue, rule resolution by specificity, versioning, every
validation the system enforces, and worked scenarios.

Includes the three accounting defects found in the live data — revenue
recognised on collection rather than accrual (36 of 42 rules), nothing
deferred at all, and 3,032,258 EGP of customer advances sitting in revenue
accounts — with the correct treatment for each.

18 mermaid diagrams, all verified to render with mermaid-cli 11.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>