SANDBOX: verify buyer -> create -> validate -> route -> transmit (mock or partner Peppol sandbox) -> signed evidence receipt

One call for the whole e-invoice flow. Body: {"invoice": <same JSON as POST /einvoice/create>, "mode": "sandbox", "transport": "mock"}. Steps: VIES check of buyer.vat_id (invalid VAT stops the flow), sanctions screen of the buyer name, EN 16931 UBL creation + validation (invalid stops the flow), route lookup (Peppol Directory, country mandate, France PPF), transmission via a pluggable transport, and an Ed25519-signed evidence receipt (JWS; public key at GET /einvoice/evidence-key). SANDBOX ONLY. transport: 'mock' (default; transmits nothing), 'recommand' or 'e-invoice-be' (the validated Peppol BIS 3 UBL is really submitted to that Peppol access point's sandbox/playground and the partner document id is returned and signed into the receipt; it is addressed to W System's own sandbox company, never to the buyer, and does not reach the production Peppol network; transmission.transmitted stays false). Partner transports switch invoice.format to peppol-bis3. mode=live returns 501 until production Peppol sending is live. Free tier shared, then x402 $0.08 (same as create).