Improve pylint compliance in formula parser

[alexanderthclark]

Summary

• Focus pylint cleanup on one module: freeride/formula.py.
• Replace the stale module docstring, remove trailing whitespace, and avoid implicit string concatenation in the quadratic term regex.
• Refactor the restricted AST evaluator to use an explicit operator map and a single return path.
• Rename internal parser temporaries so pylint no longer reports coefficient/point names in this module.

Impact

• No public API changes: _formula(...) and _quadratic_formula(...) keep the same inputs and outputs.
• This does not attempt to fix repo-wide pylint. It makes one module clean under the current local pylint version.

Testing

• python -m pylint freeride/formula.py -> 10.00/10
• pytest -q tests/test_formula.py -> 21 passed
• pytest -q -> 136 passed