Refactor iterable handling for seq

[t-kalinowski]

Summary

• centralize iterable seq lowering and keep switch-based structure
• reuse shared iterable length logic in sapply/for contexts
• add seq-like iterable tests for indexing and sapply validation

Testing

• air format .
• R -q -e 'devtools::test()'
• R -q -e 'rcmdcheck::rcmdcheck(error_on = "warning")'

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.55%. Comparing base (7746497) to head (f3371af).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #76      +/-   ##
==========================================
+ Coverage   85.22%   86.55%   +1.33%     
==========================================
  Files          16       16              
  Lines        3012     3139     +127     
==========================================
+ Hits         2567     2717     +150     
+ Misses        445      422      -23     

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