Correct invoice approval GL posting to standard accounting

[dotnetspark]

Summary

The current invoice approval flow posts GL entries in the reverse of standard accrual accounting.

Problem

The implementation currently debits AP_CONTROL and credits the expense account during invoice approval.

Expected behavior

For a standard approved vendor invoice, approval should:

• debit the vendor-category-derived expense account
• credit AP_CONTROL

Scope

• update the GL posting rule
• update approval tests
• update repo-owned specs/docs/ADR references to match the corrected accounting behavior
• preserve original_assignment.md as the historical prompt source

Notes

This intentionally prioritizes accounting correctness over the original interview prompt's reversed debit/credit example.