fix: preserve explicitly provided zero carbon intensity

[nawaaaaaAaar]

Fixes #1440.

Treat a numeric zero carbon-intensity value as a provided factor, not as missing data. This applies both to direct country intensity and to zero-valued source factors when accounting for covered generation.

No shipped factor values, accounting definitions, or fallback behavior for genuinely missing source factors are changed.

Verification on Python 3.12.13:

• Six new cases: four regressions fail before the fix, two positive-factor controls pass.
• Package test command: 686 passed, two skipped, three integration cases deselected, ten subtests passed.
• All applicable autoflake/isort/Black/flake8 hooks passed.

Zero-valued source factors are tested with a controlled factor-table fixture. The package test scope excludes visualization and API/dashboard integration; it is not a scientific validation of alternative factor tables.

AI disclosure: autonomously implemented and tested using Perplexity Computer (GPT-6.1). No human review is claimed.

Combined with #1439 and #1443: 695 passed, two skipped, three integration cases deselected and twelve subtests passed. PR is open and ready for review; hosted verification workflows require maintainer approval. A successful PR-size labeler is not a hosted test pass.

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[davidberenstein1957]

Correct fix: a zero source factor is a value, not a missing one, so is not None is the right test in both places in core/emissions.py. total_TWh still resolves to None and is skipped, so nothing else changes. Tests included. Fixes #1440.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.30%. Comparing base (7929a4f) to head (57ee053).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1441      +/-   ##
==========================================
+ Coverage   92.07%   92.30%   +0.23%     
==========================================
  Files          49       49              
  Lines        5199     5199              
==========================================
+ Hits         4787     4799      +12     
+ Misses        412      400      -12     

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