Implement RNA degradation ratio calculation

## Summary Add PEPPRO's RNA degradation ratio metric for assessing RNA sample integrity. ## Features to implement - Insert size distribution calculation (single-end and paired-end) - Degradation ratio: (10-20nt fragments) / (30-40nt fragments) - Flash integration for paired-end overlap detection - Insert size distribution plotting ## Implementation details - Create modules/local/rna_integrity - Use cutadapt output for single-end calculations - Integrate flash for paired-end fusion - Generate insert size distribution plots ## Technical specifications - Target degradation ratio: <1.0 for high-quality libraries - Support both single-end and paired-end data uniformly - Account for UMI length in calculations ## Acceptance criteria - [ ] Degradation ratio calculation for both data types - [ ] Insert size distribution plots - [ ] Integration with QC reporting - [ ] Proper UMI length accounting ## References - PEPPRO RNA integrity methods