Expand rates vignette

[pwinskill]

Summary

• expand the example vignette
• add dedicated section explaining ft_sev
• explain units versus time aggregation

Testing

• devtools::document() (fails: bash: command not found: R)
• devtools::test() (fails: bash: command not found: R)
• devtools::check() (fails: bash: command not found: R)

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https://chatgpt.com/codex/tasks/task_e_687f4e3396e8832696d528b9cbcfcc17

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.69%. Comparing base (1335cf7) to head (35d7c73).

Additional details and impacted files

@@             Coverage Diff             @@
##           severe_inc       #9   +/-   ##
===========================================
  Coverage       87.69%   87.69%           
===========================================
  Files               6        6           
  Lines             195      195           
===========================================
  Hits              171      171           
  Misses             24       24           

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