CRAN review fixes

[oneilsh]

• CRAN resubmission. Addresses reviewer feedback:
  • Added a reference to Hinze and Paterson (2006) doi:10.1017/S0956796805005769 in DESCRIPTION.
  • Added missing \value tag for print.flexseq().
  • Removed \dontrun{} example wrappers from internal helpers.
• Removed the plot() in favor of plot_structure() as part of the developer API.
• Added inst/CITATION with a Zenodo DOI for the package.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.84%. Comparing base (d7819ea) to head (0c4b6b8).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main       #8    +/-   ##
========================================
  Coverage   88.83%   88.84%            
========================================
  Files          55       55            
  Lines        5052     4929   -123     
========================================
- Hits         4488     4379   -109     
+ Misses        564      550    -14     

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