expression-parsing: add notation_warnings and NotationWarning

[m-messer]

Moves notation-warning detection out of compareExpressions' evaluation.py into lambdafeedback.expression_parsing. Independent of the criteria stack.

Changes

• notation_warnings(response, strict_syntax) in the new notation.py returns the NotationWarnings for a response:
  • EXPONENT (^) and FACTORIAL (!) under strict syntax;
  • TRIPLE_FACTORIAL (!!!) always.
• NotationWarning is a StrEnum whose values are compareExpressions' feedback tags (NOTATION_WARNING_EXPONENT, ...), so a tag lookup keeps working. Wording the warnings stays with the caller.
• Same behaviour as before: the checks are substring tests, so under strict syntax != also warns FACTORIAL, and !!! warns both FACTORIAL and TRIPLE_FACTORIAL. Documented in the docstring and CHANGELOG.

Downstream
Additive. compareExpressions can call this and keep its existing tag-keyed feedback text.

Verification

• ruff, format and mypy are clean, and all four test suites pass.
• New tests cover each warning in strict and natural mode, the != and !!! cases, and that the enum values match the compareExpressions tags.

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