Keep reactant-less v0 photolysis as an emission

[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, e.g.:

{ "type": "PHOTOLYSIS", "__music_box_type": "EMISSION", "MUSICA name": "EMIS_NO",
  "reactants": {}, "products": {"NO": {"yield": 1}, "irr__...": {"yield": 1}} }

In the CB05 MBI config, 14 of 39 photolysis reactions are reactant-less; parse() returned only the 25 with reactants, so those emissions (and their irr accumulators) vanished. Downstream this breaks a music_box run: the conditions CSV still supplies their rates → User-defined rate parameter PHOTO.EMIS_NO not found in the mechanism.

Fix

A reactant-less photolysis is a pure production term, so parse it into the Emission list (which has no reactant) instead of dropping it. This keeps the reaction and its irr__ product. Reactions with a reactant are unchanged. Adds a v0 photolysis test case (a reactant-less reaction is now asserted to appear as an emission). All 12 v0 tests pass.

Addresses NCAR/musica#986.

Follow-ups for the full pipeline

These reactions become emissions, so their MICM rate-parameter label changes from PHOTO.<name> to EMIS.<name>. To run a converted MBI config (e.g. CB05) end-to-end, two more pieces are needed:

1. a musica release picking up this mechanism_configuration change;
2. the music_box config converter remapping the PHOTO.<name> rate-parameter CSV columns to EMIS.<name> for reactions that are emissions.

(An alternative design — making Photolysis.reactants a vector to allow zero reactants while keeping the PHOTO. label — was rejected here because it changes the Photolysis type and would break musica's bindings, requiring an atomic cross-repo release. Routing to Emission is self-contained.)

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.42%. Comparing base (9ca3910) to head (a9f1d47).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #311      +/-   ##
==========================================
+ Coverage   88.08%   88.42%   +0.34%     
==========================================
  Files          44       44              
  Lines        2442     2661     +219     
==========================================
+ Hits         2151     2353     +202     
- Misses        291      308      +17     

Files with missing lines	Coverage Δ
src/v0/photolysis_parser.cpp	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

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[K20shores]

Superseded by #312: keep reactant-less reactions as photolysis (vector reactants) instead of routing to emission, so the PHOTO. rate-parameter label is preserved and no converter remap is needed.