interpolate: periodic scalar splines ignore x_min in wrapping

[krystophny]

Periodic scalar spline evaluation currently wraps x via modulo(x, period) instead of modulo(x - x_min, period) + x_min. This breaks periodic behavior when x_min != 0 and the caller evaluates outside [x_min, x_max].

Affected (scalar) entry points:

• evaluate_splines_1d / _der / _der2
• evaluate_splines_2d / _der
• evaluate_splines_3d / _der / _der2

Expected:

• For periodic splines, x should be wrapped relative to x_min: xj = modulo(x - x_min, period) + x_min.

Status:

• Fixed and covered by tests in PR GPU spline many-point API + OpenACC (benchmarks in bench/spline_many) #200 (adds periodic wrap tests that evaluate outside the domain with nonzero x_min).