Extract a diagonal from a 2-D matrix or construct a 2-D matrix from a 1-D diagonal vector.

Follows numpy.diag semantics:

• If v is 1-D of length $N$, output is 2-D of shape $(N + |k|, N + |k|)$ with v on diagonal $k$ and zeros elsewhere.
• If v is 2-D, output is 1-D containing entries v[i, i + k] for valid $i$.