Add emissions configuration schema (inventories, species maps, sources) to mechanism v1

[smeechncar]

Summary

Closes #272.

Adds a new emissions::v1 namespace to MechanismConfiguration that
parses YAML-based emissions configs for consumption by MUSICA/MIEM.

What's new

• include/mechanism_configuration/emissions/v1/types.hpp — public types
  (EmissionsConfig, SourceDescriptor, Inventory, SpeciesMap, etc.)
• include/mechanism_configuration/emissions/v1/parse.hpp — public API:
  ParseEmissions(path) and ParseEmissions(string)
• src/emissions/v1/parser.cpp — full implementation (yaml-cpp private,
  not exposed in headers); supports ${VAR} / ${VAR:-default} env var
  expansion in data root
• 12 new ErrorCode values in errors.hpp / errors.cpp
• 14 unit tests covering valid configs, all error paths, and optional-section
  handling

Design notes

Follows the existing v1 chemistry parser pattern: std::expected<T, Errors>
return type, ErrorCode enum errors, yaml-cpp kept private to the library.
Three intentional differences from the MICM parser shape are noted for
reviewers:

1. No Parse(YAML::Node) overload — emissions configs are always file/string
   entry points
2. No separate CheckSchema()/Build() split — validation and construction
   are a single pass
3. Types live in emissions::v1::types (deeper namespace) to avoid collision
   with mechanism_configuration::types

Aside for MC maintainers

While reviewing this I noticed that yaml-cpp headers currently leak into
MC's public headers in a few places (direct YAML::Node parameters on
public APIs). This isn't blocking for MIEM (MIEM only links against MUSICA,
not MC directly), but it could cause issues for downstream consumers. Flagging
here in case the maintainers want to address it in a follow-up.

Test plan

• All 14 new emissions tests pass
• All 19 existing chemistry tests still pass (no regressions)
• Build tested on Derecho with -DMECH_CONFIG_USE_FMT=ON (Intel LLVM,
  no <format> header)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 31.57%. Comparing base (04d2f08) to head (7dd4271).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #281   +/-   ##
=======================================
  Coverage   31.57%   31.57%           
=======================================
  Files           4        4           
  Lines          19       19           
=======================================
  Hits            6        6           
  Misses         13       13           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[smeechncar]

Next steps (for context)...

This PR is the first of three pieces. Once merged, the plan is:

1. MUSICA — add a translator (musica::emissions::Translate) that calls ParseEmissions, walks the resulting EmissionsConfig, and constructs MIEM's EmissionsBuilder / Source objects. MUSICA is the only layer that sees both MC and MIEM types.
2. MIEM integration test — an end-to-end test using a real CAMS-GLOB NetCDF file to verify the full config → translate → runtime pipeline.

Flagging so reviewers know the public types in emissions::v1::types and the two ParseEmissions overloads are the stable interface MUSICA will depend on.

[K20shores]

We want the description of emissions to be agnostic of the miem tool. I think it would be better to include a new section in the mechanism object called Emissions and put all created structs into that.

This would mean our v1.x format would have a new keyword, emissions, which essentially takes lines 5-end of examples/emissions/v1/emissions.yaml and indents it under the emissions key.

I guess I'll think about this a bit longer before adding more specific comments.

[boulderdaze]

Would you mind updating the PR title? We include PR titles in the release notes, so we prefer them to be more descriptive and easier to read

[K20shores]

Yeah, we need to rethink this design. We want emissions to be part of the mechanism configuration, not a separate parser. TO do that, emissions, should be part of the mechanism, not its own file. We also need to support both yaml/json so sheel expansion needs to be dropped.

[smeechncar]

OK that was a bit of a rewiring but I think this latest commit will address this. We restructured to fold emissions into the mechanism: types moved into mechanism_configuration::types, EmissionsConfig added as an optional field on Mechanism, and emissions is now parsed as a section of the main Parse() call rather than a standalone parser. Dropped data_root and shell expansion for JSON parity. The separate emissions::v1 namespace and ParseEmissions() API are gone.

[K20shores]

we do not want a separate namespacde for emissions

[smeechncar]

emissions::v1 namespace is gone. Types are now in mechanism_configuration::types, EmissionsConfig is an optional field on Mechanism, and emissions is parsed as a section of the main Parse() call. Dropped data_root and shell expansion for JSON parity. The standalone ParseEmissions() API and separate example file are removed.