EN 16931 BT annotations in codec spec docs

[ignromanov]

Part of #8 · Layer 3 (docs) · 🟢 P2 · v1.2 docs (zero code cost)

What

Annotate the codec/invoice field model with EN 16931 business terms (BT-*) in the spec docs, identifying which fields map to standard BTs and which crypto-native fields (network_id, token_address, decimals, wallet, salt) have no EN 16931 home (→ EXTENDED vendor namespace).

Why

~55–60% of EN 16931 mandatory BTs already map to our fields. Annotating now (docs only) prevents a costly retrofit when UBL/CII export lands in the merchant track.

Acceptance

• field → BT mapping table in docs
• gap fields + defaults documented (BT-3 380, BT-130 C62, VAT category, ISO 3166 country split)
• crypto-native fields flagged as EXTENDED vendor-namespace candidates

wiki: synthesis/einvoicing-standards-void-layer-verdict.md, concepts/en16931-void-layer-field-mapping.md