Task: Codegen field mappers from entity models

Deferred: generate the per-entity field-string mappers from the entity org models, which already know field names, labels and types. Only after the hand-written currency shape is proven and the codegen org-model migration is complete.

• Mappers generated for all entities with history support.
• Hand-written mappers retired.
• Zero-diff guardrails green.

1. Add a mapper template to ores.codegen mirroring the hand-written currency mapper.
2. Derive labels/formatting from model properties; extend models where display metadata is missing.
3. Replace hand-written mappers with generated ones, zero-diff where the hand-written shape was followed.