JenkinsLookup3.decode can repeat a one-byte payload across an oversized output #894

JenkinsLookup3.decode can repeat a one-byte payload across an oversized output array:

import numpy as np
from numcodecs import JenkinsLookup3

codec = JenkinsLookup3()
out = np.zeros(4, dtype="u1")
codec.decode(codec.encode(b"x"), out=out)
print(out.tobytes())  # b'xxxx'

The checksum is valid for b"x", but the four bytes returned to the caller are different. The codec API requires an output buffer of exactly the decoded size, so I expected a size error before any write.

Correctly sized buffers fail in the other direction. With a 12-byte payload, an output array of shape (2, 3) and dtype uint16 raises a broadcasting error, and a 12-byte bytearray raises an AttributeError. All three cases go through the Jenkins decoder's direct byte-view assignment.

Reproduced using the NumCodecs 0.17.1 and NumPy 2.5.4 wheels on CPython 3.12.11, macOS arm64. Codex assisted with reproduction.