Function validateChart

validateChart(chart: RpeJson): ValidationIssue[]
Validate an RPE chart for common structural issues.

Checks performed:
- BPM list is non-empty and all BPM values are positive.
- father references are in range and not self-referential.
- No circular parent chains.
- Lines have at least one event layer.
- Notes have valid type (1–4), above (1 or 2), and non-zero hold length.
Parameters
- chart: RpeJson
  The RPE chart.
Returns ValidationIssue[]
Array of ValidationIssue objects (empty = no issues).
Example
```
const issues = validateChart(chart);
const errors = issues.filter(i => i.severity === 'error');
if (errors.length > 0) {
  console.error('Chart has errors:', errors);
}
```
- Defined in chart-utils.ts:1713