feat(transforms): add cosmological redshift and Doppler shift support (Issue #28)

[Akshat1000Sharma]

Closes #28

Summary

This PR adds cosmological redshift and Doppler shift support to Spectra.jl, including mutating and non-mutating APIs. It also fixes a Unitful velocity conversion issue in this implementation, adds comprehensive tests, and updates docs. The full test suite passes with no regressions.

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Issue

Issue #28 (“Tokyo Shift”) requested redshift functionality. Spectra.jl previously had no API to shift the spectral axis by redshift z or radial velocity v.

During implementation, I identified and fixed a Unitful bug where velocities in km/s were incorrectly scaled due to stripping units without explicit conversion to m/s.

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Resolution

Added

• redshift, redshift!
• doppler_shift, doppler_shift!
• Documentation in transforms.md
• 45 new tests (formulas, Unitful support, @inferred, roundtrip, metadata, mutability)

Fix

Explicit Unitful conversion before stripping:

_velocity_ratio(v::Quantity) = ustrip(u"m/s", v) / _c_mps

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Review Commands

Run full test suite:

julia --project -e 'using Pkg; Pkg.test()'

Run only redshift tests:

julia --project -e 'using Pkg; Pkg.test("Spectra"; test_args="--testset transforms/redshift")'

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Test Results

Test Summary: | Pass  Total   Time
  Overall     |  269    269  57.1s
    SUCCESS
     Testing Spectra tests passed

All 269 tests pass (including 45 new redshift tests).
No regressions.

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Please let me know if any adjustments are needed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.71%. Comparing base (58828d3) to head (9d8d246).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #46      +/-   ##
==========================================
+ Coverage   90.74%   91.71%   +0.97%     
==========================================
  Files           8        9       +1     
  Lines         162      181      +19     
==========================================
+ Hits          147      166      +19     
  Misses         15       15              

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