This PR adds cubic interpolation support to the branch where only lower-order interpolation was previously available. The change introduces a cubic path in the interpolation flow while preserving existing behavior for current modes.

• Interpolation mode extension

  • Adds a cubic branch to the interpolation selector/dispatch logic.
  • Keeps existing linear/quadratic behavior unchanged when cubic is not requested.

• Cubic evaluation path

  • Implements cubic coefficient/evaluation handling in the interpolation routine.
  • Reuses existing data flow and interfaces so callers do not need API changes.

• Coverage updates

  • Extends tests around interpolation mode selection and expected interpolated values for cubic inputs.

Example usage (new mode path):

y_interp = interpolate_branch(x, y, mode="cubic")

This routes through the new cubic branch and returns values using cubic interpolation, while other modes continue to use their existing implementations.

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