docs: add POS + Cash Register implementation plan (3 of 5)
9-task plan covering:
- POS engine (sale processing, Decimal totals, margin info)
- Customers blueprint (CRUD, credit, vehicle history, statements, payments)
- POS blueprint (sales, quotations, layaways with stock reservation)
- Cash register (open/close, X/Z cuts, daily consolidated summary)
- POS frontend (F1-F6 shortcuts, margin display, payment modal)
- Customers frontend (search, credit status, account statements)
- Full integration test (20+ tests covering entire sale lifecycle)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
consultoria-as
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