fix(#50): complete GS1 DataBar rewrite per ISO/IEC 24724

Replace placeholder stub with full combinatorial encoding from Annex B:
- Omnidirectional: 46 elements, 96 modules, mod 79 checksum
- Limited: 47 elements, 89 finder patterns, mod 89 checksum
- Expanded: binary data encoding with numeric/alpha/ISO 646 modes,
  Method 1 (AI 01 compressed) and Method 2 (general), mod 211 checksum

Includes combins(), getWidths(), interleaveWidths() core algorithms,
all lookup tables from the spec, and proper GTIN check digit validation.

30 tests covering structural correctness.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: productdevbook