docs(spec): clarify placement of vat computation

Co-authored-by: Orca <help@stably.ai>

docs/superpowers/specs/2026-08-06-supplier-po-pdf-vat-design.md

@@ -85,8 +85,8 @@ PurchaseOrderDocument(props)
    > Different from the quote (where VAT is added to the customer's
    > selling price); this is the buy-side equivalent.
 
-3. **Computations** (just below the existing `subtotal` block,
-   ~lines 97–110) — add:
+3. **Computations** (just after the existing
+   `totalAmount` snapshot reconciliation, ~line 110) — add:
 
    ```ts
    // 15% VAT on top of the supplier-quoted subtotal. Uses the same