Enforce exclusive variable computation modes

[MaxGhenis]

Summary

• reject variable classes that combine formula, adds/subtracts, and class-level uprating modes
• make runtime variable.uprating = ... assignments validate the same invariant, rolling back invalid assignments
• make @uprated fail fast when applied to classes that already define a formula, aggregation metadata, or uprating metadata
• remove the obsolete formula fallback path in @uprated now that uprating is input-only, and cover the new invariant with tests

Verification

• red-state check before implementation: new mixed-mode tests failed as expected
• red-state check for runtime uprating mutation: failed before adding the setter guard
• uv run --no-sync pytest tests/core/variables/test_variables.py tests/core/test_medium_fixes.py -q
• uv run --no-sync ruff check policyengine_core/variables/variable.py policyengine_core/commons/formulas.py tests/core/variables/test_variables.py tests/core/test_medium_fixes.py
• uv run --no-sync ruff format --check policyengine_core/variables/variable.py policyengine_core/commons/formulas.py tests/core/variables/test_variables.py tests/core/test_medium_fixes.py
• uv run --with towncrier towncrier build --draft --version 3.26.8

Depends on country cleanup/releases before merge:

• Remove mixed computation metadata from variables policyengine-uk#1677
• Remove mixed computation metadata from variables policyengine-us#8330