Add three-layer TrustLayer architecture

[fabaleus]

Proposes a transport-independent trust layer for cross-organization business document exchange, anchored on the European Business Wallet (EUBW) ecosystem. The same trust artifacts can ride on Peppol AS4, mail, QERDS, or other transports.

Three layers:

• Layer 1 (envelope): the Portable Trust Envelope itself — STM (Sealed Trust Manifest), qualified electronic seal, TVE (Trust Validation Envelope). Transport-agnostic data model.
• Layer 2 (protocol): APEIP, the AP-to-EUBW Integration Protocol. Used identically by all transport bindings.
• Layer 3 (bindings): one document per transport, specifying the carrier element, the handler role, the seal format, and discovery.

Layout under TrustLayer/:

• README.md: orientation, three-layer model, navigation.
• architecture/PTE-Architecture.md: the Portable Trust Envelope specification. STM + qualified seal + TVE. Peppol binding used as the worked example throughout, with binding-specific details marked as such.
• protocol/APEIP-Specification.md: the AP-to-EUBW Integration Protocol. Outbound endpoints (/credentials/for-document, /seal, /presentation-request) and inbound endpoint (/verify). Auth modes, error model, versioning.
• bindings/README.md: the binding matrix.
• bindings/peppol/Oxalis-NG-Handler.md: the Peppol transport binding, with Oxalis-NG as the reference implementation platform. EUBW-targeted via APEIP.
• bindings/mail/Mail-Binding.md: sketch of the mail (SMTP/IMAP) binding using multipart/related MIME structure.
• bindings/qerds/QERDS-Binding.md: sketch of the QERDS (eIDAS Art. 44) binding for legal-evidentiary scenarios.
• sc5-binding/SC5-PTE-Technical-Binding.md: mapping of SC5 attestations (Approved Supplier, Authorized Service Provider) onto the PTE, with mermaid sequence diagrams for Scenarios 1, 2, and 3. Closes the deferred technical questions in SC5 (where attestations ride in the AS4 exchange, how they bind to invoice content, how the verification result reaches C4, how missing credentials are pulled at receive time).

All documents are drafts for discussion. The Peppol binding is the most developed (SC5 Pilot 1 proposed as first vehicle). Mail and QERDS bindings are sketches demonstrating transport-independence; they are not pilot-ready.

[kjorlaug]

A lot of interesting thoughts here - but the discussion belongs in the architecture repos, not here in SC5. Take a look at this for instance: webuild-consortium/wp4-architecture#261