test: add negative tests for custom metrics (CustomDefine/CustomUndefine)

Add custom_metric_test.go with 12 tests covering:
- Define with factor=0 (valid, produces 0 in base units)
- Define with negative factor (valid)
- Malformed input (double-space collapse causes parse error)
- Define with existing metric name (error: duplicate)
- Custom metric in arithmetic (+, -, *, /)
- Custom metric in hyper operations ([+])
- Cross-category conversion failure (Custom -> DataRate)
- Cross-category operation failure (Time + Distance)
- Custom metric display in Show

Each test properly cleans up defined custom metrics to avoid
polluting the global MetricRegistry.

0 files changed, 0 insertions, 0 deletions