Refactor legacy pure-function logic

[charles-turner-bot]

Summary

• extract a small set of logic-bearing pure helpers from the legacy camelCase path into src/legacy_pure_functions.jl
• reuse those helpers from partial-cruise and exception-handling code without changing external behaviour
• add unit tests covering the extracted pure functions and wire them into the package test suite

Testing

• docker run --rm -v "$PWD":/work -w /work julia:1.11 julia --project=. -e 'using Pkg; Pkg.test()'

Notes

• this intentionally avoids turning the legacy path into a pile of tiny wrappers; only the chunky testable logic was extracted
• existing package precompile warnings about duplicate method definitions were observed during test setup but the full suite still passed (65/65)

[codecov-commenter]

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Codecov Report

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@@            Coverage Diff             @@
##            main      #13       +/-   ##
==========================================
+ Coverage   7.82%   20.09%   +12.27%     
==========================================
  Files         16       17        +1     
  Lines       1023     1030        +7     
==========================================
+ Hits          80      207      +127     
+ Misses       943      823      -120     

Files with missing lines	Coverage Δ
src/GLODAP_Section_Gridder.jl	100.00% <ø> (ø)
src/exception_handling.jl	24.24% <100.00%> (+24.24%)	⬆️
src/legacy_pure_functions.jl	100.00% <100.00%> (ø)
src/partial_cruises.jl	14.94% <100.00%> (+14.94%)	⬆️

... and 3 files with indirect coverage changes

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