[P1] GRE modal 01 (transporte público) + Transportista doc 31

[Railly]

Context

PR #7 shipped GRE Remitente (tipoDoc=09) with modTraslado=02 (transporte privado, emisor mueve los bienes). When the customer hires a third-party carrier (modTraslado=01) or when the carrier itself emits the guía (tipoDoc=31), the schema diverges.

Scope

1. Modal 01 (transporte público)

   • Add `cac:CarrierParty` block to `src/cpe/ubl/gre.ts`
   • Validate transportista RUC + nroMtc (registro MTC)
   • Make chofer optional (carrier provides their own)

2. GRE Transportista (tipoDoc 31)

   • New UBL builder `src/cpe/ubl/gre-transportista.ts` (different schema entirely)
   • New emit path in REST client (same endpoint, different content)

3. Multiple choferes — schema accepts loop, currently 1-only

Success criteria

• Modal 01 verified against SUNAT beta
• Transportista (31) verified against SUNAT beta
• Unit tests for each new schema variant
• LIMITATIONS.md: 4 sub-bullets in GRE section move from 🚧 to 🔬 or ⚠️

References

• LIMITATIONS.md → "Guía de Remisión Electrónica (GRE)" sub-limitations
• Greenter despatch2022.xml.twig has the full modal 01 + carrier blocks