fix: std.round preserves identity for |x| >= 2^52

[He-Pin]

Motivation

std.round returned the wrong integer for large odd inputs in
[2^52, 2^53). For example, std.round(9007199254740991) (2^53 - 1)
produced 9007199254740992. The prior floor(x + 0.5) /
ceil(x - 0.5) implementation invokes IEEE 754 round-to-even when the
ULP is >= 1.0, so adding 0.5 rounds to the nearest even integer
rather than the correct half-away-from-zero value. go-jsonnet
(math.Round) and jrsonnet both return the identity for these inputs;
sjsonnet was the outlier.

Modification

• Short-circuit in MathModule.scala std.round when |x| >= 2^52
  or x is NaN — at that magnitude every double is already an exact
  integer (ULP >= 1.0), so returning x unchanged is both correct and
  avoids the precision trap.
• Half-away-from-zero semantics are preserved for all smaller
  magnitudes; NaN / Infinity propagate naturally.
• Add five regression assertions (2^53-1, 2^53-2, 2^52+1, -(2^53-1),
  1e20) to the existing std.round/isEven/isOdd/isInteger/isDecimal
  fixture.

Result

std.round now matches go-jsonnet and jrsonnet on every integer in
[2^52, 2^53). The existing std.round(0.5)==1 /
std.round(-0.5)==-1 half-away-from-zero semantics are preserved.
All 23 file tests plus 86 evaluator tests pass.

References

• IEEE 754-2019 §4.3.1: Rounding-direction attributes
  (roundTiesToEven)
• go-jsonnet math.Round: https://github.com/google/go-jsonnet/blob/master/builtin.go
• Jsonnet spec: https://jsonnet.org/ref/stdlib.html#math
  (std.round "ties away from zero")