Extension point for host-specific line fields

[jbagsik]

Part of

Field fidelity and validation of parsed e-invoices — restoring facts the parser reads but the model cannot hold, and making the readiness figure agree with the conformance verdict.

What to build

A host can contribute its own line fields — through persistence, validation and display — without those fields living in the generic package. The new path is added beside the existing one so nothing breaks; migrating the existing fields follows separately.

Acceptance criteria

• A host can register additional line fields and have them persisted, validated and displayed
• The existing hard-coded fields keep working unchanged while the new path exists
• The extension point is documented in the package README
• A test covers a host contributing one additional field end to end
• Where the extension point reaches the invoice header, a field the model cannot accept fails the same way it does on a line — one rule, not two behaviours

Blocked by

• The line draft never discards a supplied field silently #4 — The line draft never discards a supplied field silently