Function linearInterpolateNaNs

Fill internal NaN values using linear interpolation between the nearest valid neighbours.

Equivalent to linear_interpolation() in timesfm_2p5_base.py.

This implementation is strictly O(n) with two single-pass traversals:

1. Forward pass: record the last valid value and its position at every index into auxiliary arrays.
2. Backward pass: for each NaN, interpolate using the pre-recorded left-valid (value & position) and a running right-valid pointer. No inner loops — each array element is visited exactly twice in total.

Edge cases:

• No NaNs → returns the original array.
• All NaNs → returns an all-zero array of the same length.
• Only one valid point → NaNs are filled with that value.