Remove mixed computation metadata from variables

[MaxGhenis]

Summary

• remove redundant uprating metadata from variables already computed by formulas or adds
• remove dead class-level adds pointers from state variables whose formulas already add the parameter-defined variable lists explicitly
• make default uprating apply only to plain input variables, so system-level mutation cannot recreate mixed computation modes
• preserve microdata dataset-extension uprating for computed columns through a data-construction-only override map, including computed variables that previously received the default AGI uprater
• keep a changelog fragment so this cleanup releases before PolicyEngine Core enforces mutually exclusive computation modes

Verification

• red-state runtime test: test_variables_use_at_most_one_computation_mode failed with 21 conflicts before the default-uprating fix
• AST scan for variables mixing formula, adds/subtracts, and uprating: conflicts=0
• runtime scan after CountryTaxBenefitSystem() under current core: conflicts=0
• runtime scan after CountryTaxBenefitSystem() with local PolicyEngine Core PR Add legislative reference for SNAP's 30% expected food contribution #495 on PYTHONPATH: conflicts=0
• default-uprated computed-variable override scan: missing_overrides=0
• uv run --no-sync pytest policyengine_us/tests/test_system_import.py policyengine_us/tests/microsimulation/data/test_extend_single_year_dataset.py -q
• PYTHONPATH=/Users/maxghenis/policyengine-core uv run --no-sync pytest policyengine_us/tests/test_system_import.py -q
• uv run --no-sync ruff check on touched variable, default-uprating, economic-assumptions, and test files
• uv run --no-sync ruff format --check on touched variable, default-uprating, economic-assumptions, and test files
• uv run --with towncrier towncrier build --draft --version 1.691.14

Companion cleanup for UK: PolicyEngine/policyengine-uk#1677. Core guard PR: PolicyEngine/policyengine-core#495.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.57%. Comparing base (8cb60e7) to head (f4a38b0).
⚠️ Report is 29 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##              main    #8330      +/-   ##
===========================================
- Coverage   100.00%   94.57%   -5.43%     
===========================================
  Files            3       14      +11     
  Lines           63      166     +103     
  Branches         0        2       +2     
===========================================
+ Hits            63      157      +94     
- Misses           0        9       +9     

Flag	Coverage Δ
unittests	94.57% <ø> (-5.43%)	⬇️

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