Add billing-only Peppol registration

[gregjotau]

Fixes #83.

This adds a backwards-compatible narrow registration path for applications that validate only Peppol Billing documents:

• initBilling on the combined European release classes
• PeppolValidationBisEurope.initBilling across supported releases
• existing init methods still register the same complete set, in the same order
• focused tests verify the exact billing-only contents and absence of post-award/order sets

Why this matters: our ReAI service selects only the invoice and credit-note validation IDs. With this API it can exclude ph-ubl23, whose 3.79 MiB model is used only by order change, order cancellation, and advanced order response, and avoid initializing all unrelated validator sets at startup.

Verification:

• all 7 phive-rules-peppol tests pass on JDK 25
• Maven verify passes
• a runtime smoke test called PeppolValidation2025_05.initBilling with ph-ubl23 physically removed from the classpath and registered exactly two validation sets