fix: error thunk propagation and std.round(-0) sign

[He-Pin]

Motivation

Two bugs found via adversarial testing against go-jsonnet and jrsonnet:

1. Error thunk false "Infinite recursion": When an error thunk was forced inside a binary operator, sjsonnet incorrectly reported "Infinite recursion detected (self-referential thunk)" instead of propagating the original error. Root cause: tryEvalCatch caught the sjsonnet.Error (which is NonFatal), returned null, and the main dispatch re-evaluated the thunk which still had its evaluating flag set to true.

2. std.round(-0) returned 0: std.round(-0) returned positive zero instead of negative zero. Both go-jsonnet and jrsonnet correctly return -0. Root cause: x >= 0 is true for -0.0 (IEEE 754: -0 == +0), so it took math.floor(-0 + 0.5) = 0.0 (positive zero).

Expression	sjsonnet master	go-jsonnet	jrsonnet	this PR
local a = error "fail"; local b = a + 1; b	Infinite recursion detected	fail	fail	fail ✅
std.round(-0)	0	-0	-0	-0 ✅
std.round(0)	0	0	0	0 ✅

Modification

Error thunk fix (Val.scala):

• Wrap LazyExpr.value's computation in try/finally to guarantee the evaluating flag is reset even when the expression throws. This allows the main dispatch path to properly re-evaluate the thunk and propagate the original error.

std.round fix (MathModule.scala):

• Add else if (x == 0) x early return before the positive/negative branches. Since 0.0 == -0.0 is true in IEEE 754, this catches both +0 and -0 and returns x preserving the original sign bit.

Result

• All 8 affected operator patterns (+, -, *, /, string concat, !, >, array concat) now correctly propagate the original error.
• True self-referential thunks (local a = a + 1; a) still correctly detected.
• std.round(-0) returns -0, std.round(0) returns 0 — both signs preserved.
• std.atan2(std.round(0), -1) > 0 is true; std.atan2(std.round(-0), -1) < 0 is true.
• All tests pass on JVM 2.12, 2.13, 3.3.