fix: missing tfb_spline export

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 74.25%. Comparing base (a0abf6c) to head (30df3a5). Report is 14 commits behind head on dev.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #116 +/- ## ========================================== + Coverage 74.09% 74.25% +0.16% ========================================== Files 28 28 Lines 1957 1958 +1 ========================================== + Hits 1450 1454 +4 + Misses 507 504 -3 ```

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[fabian-s]

:see_no_evil: oh no, thank youuu