feat(types): add an optional invalid value hook

[MilesCranmerBot]

Add optional TypeSpec(init_invalid="() -> value") to define DynamicExpressions.invalid_value(T). The result must have the generated type and fail is_valid. Types without invalid states can omit the hook.

Require SymbolicRegression ~2.4.1, which requires DynamicExpressions ~2.11 and supplies this API transitively. Existing positional TypeSpec arguments retain their positions.

Verification

• Ordinary registry resolution selected DynamicExpressions 2.11.0 and SymbolicRegression 2.4.1, without local source overrides.
• Both renamed hook tests and two rejection subtests passed, including fit/predict.
• A failed convenience evaluation used init_invalid; the old invalid keyword was rejected.
• Pre-commit checks passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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