Convert kinetics & equilibrium code to use std::span

[speth]

Changes proposed in this pull request

This PR continues the work started in #2077 toward implementing Cantera/enhancements#237, updating the classes that are in the kinetics and equil directories.

AI Statement (required)

• Extensive use of generative AI.
  Significant portions of code or documentation were generated with AI, including logic and implementation decisions. All generated code and documentation were reviewed and understood by the contributor. Primarily done using GPT-Codex-5.2 within the Codex extension for VS Code.

Checklist

• The pull request includes a clear description of this code change
• Commit messages have short titles and reference relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• Style & formatting of contributed code follows contributing guidelines
• AI Statement is included
• The pull request is ready for review

[codecov]

Codecov Report

❌ Patch coverage is 85.50984% with 81 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.47%. Comparing base (93f8c18) to head (3c14653).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
include/cantera/kinetics/Kinetics.h	0.00%	24 Missing ⚠️
src/equil/vcs_solve_TP.cpp	75.00%	14 Missing and 1 partial ⚠️
src/kinetics/InterfaceKinetics.cpp	64.86%	12 Missing and 1 partial ⚠️
src/equil/MultiPhase.cpp	45.45%	12 Missing ⚠️
src/fortran/fct.cpp	42.85%	4 Missing ⚠️
src/equil/vcs_solve.cpp	85.00%	3 Missing ⚠️
samples/f77/demo_ftnlib.cpp	66.66%	2 Missing ⚠️
src/equil/MultiPhaseEquil.cpp	86.66%	1 Missing and 1 partial ⚠️
include/cantera/kinetics/Falloff.h	87.50%	1 Missing ⚠️
include/cantera/kinetics/ReactionData.h	0.00%	1 Missing ⚠️
... and 4 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2085      +/-   ##
==========================================
+ Coverage   77.43%   77.47%   +0.04%     
==========================================
  Files         451      451              
  Lines       52714    52744      +30     
  Branches     8838     8830       -8     
==========================================
+ Hits        40819    40866      +47     
+ Misses       8926     8911      -15     
+ Partials     2969     2967       -2     

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[ischoegl]

Thanks, @speth!