test: add comprehensive tests for logParser.worker.js

Implements Week 1 priority testing for the Web Worker parsing logic.

Coverage improvements:
- Worker: 0% → 100% statement coverage, 95.45% branch coverage
- Overall: 85.32% → 91.09% statement coverage
- Added 33 new test cases

Test coverage includes:
- Keyword-based value extraction
- Regex-based value extraction
- Step number extraction (custom keywords, case-insensitive)
- Scientific notation and negative numbers
- Metric naming (derived from name/keyword/regex)
- Error handling (empty content, invalid regex, NaN values)
- Edge cases (special characters, large numbers, mixed line endings)
- Real-world log formats (PyTorch, TensorFlow, JSON, wandb)
- Performance scenarios (1000+ data points)

This addresses the critical gap in testing the core parsing engine
that processes all user-uploaded log files.

Author: claude