Add practical genome tests using real NCBI reference sequences

Exercises generate mode end-to-end with Lambda phage, S. aureus, and
E. coli genomes across singleplex, multiplex, mixed-read, parallel,
and timing-model scenarios. Includes user-facing example and 11-test
pytest suite that skips gracefully without the datasets CLI.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: druvus