feat: typed, documented constants for message types and variants (#39)

[benjamineckstein]

Closes #39

Summary

Adds typed, self-documenting constants for SEPA message types and variants as a single source of truth. Exports MessageType (credit transfer vs direct debit), CreditTransferVariant (pain.001.* versions), and DirectDebitVariant (pain.008.* versions) as as const objects with derived type aliases. Constant-based narrowing is now possible alongside the still-supported raw string literals, enabling clearer intent in code like writeDirectDebit({ variant: SDD_V08 }) instead of magic strings. Modern defaults named SCT_V09 / SDD_V08 (version-counter mnemonics).

Usage Examples

Before: raw strings, no constants

const doc = writeCreditTransfer(model, { variant: 'pain.001.001.09' });

After: with constants (or still with raw strings, both work)

import { SCT_V09, SDD_V08 } from 'sepa-xml-ts';

const doc = writeCreditTransfer(model, { variant: SCT_V09 });  // constant-based, narrowed
const dd = writeDirectDebit(model, { variant: SDD_V08 });      // type-safe defaults
const legacyDoc = writeCreditTransfer(model, { variant: 'pain.001.001.03' });  // raw strings still work

Notes

• NON-BREAKING: resolved string-literal unions are byte-identical to raw strings; all existing code continues to type-check without modification.
• 687 tests green, clean typecheck.
• Quality review verdict: SHIP (cosmetic non-blocking notes only).

Testing

All tests pass locally:

pnpm test       # 687 green
pnpm typecheck  # clean
pnpm build      # clean

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