fix: preserve negative zero in std.round

[He-Pin]

Motivation

std.round(-0) returned positive zero in sjsonnet, while go-jsonnet and jrsonnet preserve the negative zero sign. This matters for programs that observe signed zero through functions such as std.atan2.

Modification

• Preserve the original input when std.round receives either +0 or -0.
• Keep the existing positive and negative rounding branches for non-zero numbers.
• Add a regression test that checks the sign of rounded zero values.

Result

Case	go-jsonnet v0.22.0	jrsonnet 0.5.0-pre99	sjsonnet before	sjsonnet after
std.round(-0)	-0	-0	0	-0
std.round(0)	0	0	0	0

std.atan2(std.round(-0), -1) now observes the negative-zero sign.

Risks

• This intentionally changes materialized output for the exact std.round(-0) case.
• Other rounding behavior is intended to remain unchanged.

[stephenamar-db]

please fix PR.

[He-Pin]

@stephenamar-db fixed