Correct rhs index in not_a_knot right boundary condition

[gistrec]

Bug

When using not_a_knot as the right boundary condition together with
a non-zero left boundary condition (first_deriv or second_deriv),
the left BC value was silently ignored

Root cause: set_points() wrote rhs[0] = 0.0 instead of
rhs[n-1] = 0.0 inside the not_a_knot right-BC branch. This
overwrote whatever the left-BC block had stored in rhs[0] a few
lines earlier

The bug did not affect the symmetric case (not_a_knot on both sides)
because rhs[0] was already 0 there, so the overwrite was a no-op.
That is also why no existing test caught it

Fix

src/spline.h, line 332: change rhs[0] to rhs[n-1]

Test

Added MixedBoundaryNotAKnot to unit_tests.cpp. It verifies that:

• second_deriv left + not_a_knot right: f''(x[0]) matches the requested value.
• first_deriv left + not_a_knot right: f'(x[0]) matches the requested value.

Both assertions fail on the unfixed code and pass after the fix