feat: refactor converter and parsers into modular structure (v0.11.0)

[stevez]

Split large monolithic files into focused, maintainable modules:

Converter Refactoring

• Split converter.ts (2,117 L) → converter/ (4 modules, 2,183 L):
  • converter/index.ts - Main CoverageConverter class (1,007 L)
  • converter/merge.ts - V8 coverage merging (82 L)
  • converter/sanitizer.ts - Source map sanitization (340 L)
  • converter/coverage-fixes.ts - Istanbul fixes (754 L)

Parsers Extraction

• Created parsers/ (6 modules, 553 L total):
  • Extracted bundler patterns from constants.ts
  • parsers/nextjs.ts - Next.js URL patterns (101 L)
  • parsers/vite.ts - Vite URL patterns (93 L)
  • parsers/webpack.ts - Webpack URL patterns (94 L)
  • parsers/sourcemap.ts - Source map patterns (92 L)
  • parsers/url-utils.ts - URL utilities (68 L)
  • parsers/index.ts - Re-exports (105 L)

Code Reduction

• Reduced constants.ts: 317 → 92 lines (-71%)
• Net production code: +411 lines (+17%)
  • Trade-off: Better organization vs. code size

Test Coverage

• Added comprehensive tests (+2,047 test lines):
  • 3 converter module tests (1,344 L)
  • 6 parser module tests (691 L)
• Coverage improved: 72.75% → 77.53% (+4.78%)
• All 678 tests passing (on main branch)
• Zero lint errors

Documentation

• Added docs/folder-structure.md for future refactoring reference

Backward Compatibility

• ✅ All exports remain unchanged
• ✅ Public API stays the same
• ✅ Users don't need to update their code
• ✅ No breaking changes

Files changed:

• 19 new files (converter/, parsers/, docs/)
• 11 modified files (imports updated)
• Build: ✅ Successful
• Lint: ✅ Zero errors