Expose synloop purge gas as process_gas_mixture multivariable stream

Adds a process_gas_mixture multivariable stream definition and uses it to expose the ammonia synloop purge gas as a structured output with mass flow, composition (hydrogen, nitrogen, ammonia, argon mass fractions), temperature, and pressure. The hydrogen_out and nitrogen_out outputs now represent only unused feedstock, separating them from the purge gas stream.

Section 1: Type of Contribution

• Feature Enhancement
  • Framework
  • New Model
  • Updated Model
  • Tools/Utilities
  • Other (please describe):
• Bug Fix
• Documentation Update
• CI Changes
• Other (please describe):

Section 2: Draft PR Checklist

• Open draft PR
• Describe the feature that will be added
• Fill out TODO list steps
• Describe requested feedback from reviewers on draft PR
• Complete Section 7: New Model Checklist (if applicable)

TODO:

• Define process_gas_mixture stream in commodity_stream_definitions.py
• Update AmmoniaSynLoopPerformanceModel to output purge gas via the new stream
• Add tests for stream definition and purge gas outputs
• Add documentation section to ammonia.md

Type of Reviewer Feedback Requested (on Draft PR)

Structural feedback: Is process_gas_mixture the right name/scope for this stream definition?

Implementation feedback: Are the 7 stream variables (mass_flow, 4 mass fractions, temperature, pressure) sufficient?

Other feedback: N/A

Section 3: General PR Checklist

• PR description thoroughly describes the new feature, bug fix, etc.
• Added tests for new functionality or bug fixes
• Tests pass (If not, and this is expected, please elaborate in the Section 6: Test Results)
• Documentation
  • Docstrings are up-to-date
  • Related docs/ files are up-to-date, or added when necessary
  • Documentation has been rebuilt successfully
  • Examples have been updated (if applicable)
• CHANGELOG.md
  • At least one complete sentence has been provided to describe the changes made in this PR
  • After the above, a hyperlink has been provided to the PR using the following format:
    "A complete thought. [PR XYZ]((https://github.com/NatLabRockies/H2Integrate/pull/XYZ)", where
    XYZ should be replaced with the actual number.

Section 4: Related Issues

Resolves #754

Section 5: Impacted Areas of the Software

Section 5.1: New Files

None.

Section 5.2: Modified Files

• h2integrate/core/commodity_stream_definitions.py
  • Added process_gas_mixture stream definition with 7 variables (mass_flow, hydrogen, nitrogen, ammonia, argon mass fractions, temperature, pressure).
• h2integrate/converters/ammonia/ammonia_synloop.py
  • Added add_multivariable_output call in setup() and purge gas stream population in compute(). Separated purge gas from unused feedstock outputs.
• h2integrate/converters/ammonia/test/test_ammonia_synloop_model.py
  • Added subtests verifying purge gas stream output shape, value ranges, and mass fraction correctness.
• h2integrate/core/test/test_commodity_stream_definitions.py
  • Added test_add_process_gas_mixture_output and test_add_process_gas_mixture_input.

Section 6: Additional Supporting Information

Purge gas molar fractions (hydrogen, nitrogen, ammonia, argon) are converted to mass fractions using all four species' molecular weights. The process_gas_mixture stream type is generic and can be reused by other components.

Section 7: Test Results, if applicable

• test_ammonia_synloop_model.py: 4 passed, 47 subtests passed
• test_commodity_stream_definitions.py: 6 passed, 18 subtests passed