[WIP] Improve variable list semantics for MIP table names

[Copilot]

• Port variable list semantics from fre-cli PR #846 (local_var reads data, target_var for MIP lookups)
• Fix omission tracking clash: CLI only raises ClickException for fatal errors (result < 0), not partial failures
• Update make_simple_varlist: non-MIP vars get empty string value, MIP vars get self-mapped value
• Update CLI varlist filter tests for new semantics
• Update make_simple_varlist unit test for MIP table filter
• INCOMPLETE: test_make_simple_varlist_mip_table_no_match test needs update — it expects None return but now gets dict with empty-string values since non-MIP vars are included instead of filtered out. Fix: update assertion to expect a dict with empty string values instead of None.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 80.95238% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
fremorizer/cmor_finder.py	71.42%	2 Missing ⚠️
fremorizer/cli.py	66.66%	1 Missing ⚠️
fremorizer/cmor_mixer.py	90.90%	1 Missing ⚠️

@@            Coverage Diff             @@
##             main      #74      +/-   ##
==========================================
- Coverage   95.03%   94.53%   -0.51%     
==========================================
  Files          10       10              
  Lines        1310     1317       +7     
==========================================
  Hits         1245     1245              
- Misses         65       72       +7     

Files with missing lines	Coverage Δ
fremorizer/cli.py	99.05% <66.66%> (-0.95%)	⬇️
fremorizer/cmor_mixer.py	91.28% <90.90%> (-0.37%)	⬇️
fremorizer/cmor_finder.py	95.69% <71.42%> (-4.31%)	⬇️

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