- po_id: corpus PO id, e.g. "PO-8801". Join key across the whole kit.

Returns the ERP-native PO envelope. Under sap_s4hana: an
API_PURCHASEORDER_PROCESS_SRV `{"d": {A_PurchaseOrder ... to_PurchaseOrderItem}}`
document with stringified decimals and /Date(ms)/ dates. Under netsuite: a REST
Record `purchaseOrder` with JSON numbers and an `item.items[]` sublist. Under
dynamics365: an OData V4 PurchaseOrderHeadersV2 entity with `@odata.etag` and
`dataAreaId`. Raises if the PO is unknown.

- po_id: corpus PO id, e.g. "PO-8801".

Under sap_s4hana returns an API_MATERIAL_DOCUMENT_SRV A_MaterialDocumentHeader
collection with a to_MaterialDocumentItem list (GoodsMovementType 101). `condition`
and `qty_damaged` from the corpus surface as item-level annotations (short-ship /
over-ship / damage / UOM-mismatch traps). Returns an empty collection if nothing was
received.

- invoice_id: corpus invoice id, e.g. "INV-01".

Under sap_s4hana returns an API_SUPPLIERINVOICE_PROCESS_SRV A_SupplierInvoice with a
to_SuplrInvcItemPurOrdRef line referencing the PO. Carries
SupplierInvoiceIDByInvcgParty for duplicate-invoice detection (the PO-8808 trap:
INV-08a and INV-08b bill the same PO). Under netsuite: a vendorBill; under
dynamics365: a VendorInvoiceHeader. Raises if the invoice is unknown.

Takes no args. A PO is "open" when its billed qty (summed across invoices) is less
than the ordered qty. Under sap_s4hana returns an OData collection
`{"d": {"results": [A_PurchaseOrder, ...], "__count": "N"}}`; under netsuite a
`{items, hasMore, offset, totalResults}` page; under dynamics365 an OData V4
`{"value": [...]}`.

- invoice_id: corpus invoice id, e.g. "INV-01".
- amount: payment amount; echoed into the staged payload for the human to confirm.
- currency: ISO code, default "USD".

Validates the invoice exists and the amount is numeric, then records a pending record
and returns {result:"pending_approval", approval_id, target_endpoint, ...}. Models SAP
API_JOURNALENTRYITEMBASIC_SRV vendor payment. THIS DOES NOT PAY ANYTHING — a human must
approve(approval_id, token) to execute. 'pay_invoice' is in policy.gated_actions, so
validation.policy_gated is true.

- po_id: corpus PO id, e.g. "PO-8801".
- new_qty: optional new order quantity.
- new_unit_price: optional new net price per unit.

Validates the PO exists and at least one change is present, records a pending record,
returns pending_approval + approval_id. Models a PATCH to API_PURCHASEORDER_PROCESS_SRV
A_PurchaseOrder (If-Match / x-csrf-token write). 'po_amendment' is gated. Executes only
on human approval.

- po_id: corpus PO id, e.g. "PO-8801".
- event_type: e.g. "Created" | "Changed".

Under sap_s4hana returns a SAP Advanced Event Mesh CloudEvents envelope on topic
sap/s4/beh/purchaseorder/v1/PurchaseOrder/<event_type>/v1 with payload {PurchaseOrder}.
Under netsuite: a SuiteScript user-event callback; under dynamics365: a Business Events
envelope. Use it to exercise event-driven wiring.