fix: reject fractional indices and step in std.slice (#797)

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Motivation

std.slice and the [a:b:c] slice operator silently truncated fractional index / end / step arguments instead of rejecting them:

std.slice([0, 1, 2], 0.9, 2.9, 1)   // was: [0, 1]
std.slice("abc", 0, 3, 1.5)         // was: "abc"

Official Jsonnet 0.22.0 rejects fractional array indices at manifestation (array index was not integer: 0.9). Per the maintainer decision in #797 ("go with the strict approach — if the function expects an int, reject anything else"), this applies strict integer semantics to both array and string slicing rather than truncating.

Modification

• Util.slice now accepts raw Option[Double] and validates integrality at a single enforcement point, erroring on NaN / Infinity / non-whole values.
• The std.slice builtin (SetModule) and the visitSlice operator (Evaluator) pass raw doubles through instead of pre-truncating to Int (which hid the fractional part before validation could see it).
• Regression tests:
  • slice_strict_integer.jsonnet — integers, negatives, null, and whole doubles (2.0) still work, for both std.slice and the [a:b:c] operator.
  • error.slice_fractional_index.jsonnet — fractional index rejected.

Result

sjsonnet.Error: [std.slice] slice index must be an integer, got: 0.9
sjsonnet.Error: slice index must be an integer, got: 0.5    # operator path

Full JVM (Scala 3) test suite passes; formatting verified with checkFormat.

Addresses #797.