Allow photolysis reactions with no reactants

[K20shores]

Problem

The v0 photolysis parser silently drops PHOTOLYSIS reactions whose reactants is empty. Music Box Interactive encodes emissions as reactant-less photolysis so they can carry an irr__ product; in the CB05 config, 14 of 39 photolysis reactions are reactant-less, so they (and their irr accumulators) vanished. A converted config then fails to run: the conditions CSV supplies rates for the dropped reactions (PHOTO.EMIS_NO not found).

Fix

Make Photolysis::reactants a vector holding at most one component, and keep reactant-less reactions instead of dropping them. The reaction stays a PHOTOLYSIS (keeping its PHOTO.<name> rate-parameter label), so downstream conditions that reference it by that label keep working with no remapping.

• types::Photolysis::reactants: ReactionComponent → std::vector<ReactionComponent>
• v0 parser keeps the reaction with its (possibly empty) reactants
• v1 parser parses reactants as a list; the schema already allowed 0-or-1 (it only errors on >1), and an empty list validates
• validate.cpp and the v0/v1 photolysis tests updated (a reactant-less v0 reaction is asserted to stay a photolysis with an empty reactants list)

All 34 tests pass.

Addresses NCAR/musica#986. Pairs with a musica change to handle the now list-valued photolysis reactants in the Python layer (getter/setter/serialize), plus a mechanism_configuration tag bump; they must be released together.

Supersedes #311 (which routed these to Emission; keeping them as photolysis avoids changing the rate-parameter label, so no converter remap is needed).

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.39%. Comparing base (9ca3910) to head (95a828f).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
src/validate.cpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #312      +/-   ##
==========================================
+ Coverage   88.08%   88.39%   +0.31%     
==========================================
  Files          44       44              
  Lines        2442     2655     +213     
==========================================
+ Hits         2151     2347     +196     
- Misses        291      308      +17     

Files with missing lines	Coverage Δ
src/v0/photolysis_parser.cpp	100.00% <100.00%> (ø)
src/v1/reactions/photolysis.cpp	93.18% <100.00%> (ø)
src/validate.cpp	87.50% <0.00%> (-3.18%)	⬇️

... and 2 files with indirect coverage changes

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