fix: round ties towards +Infinity in roundTo to match round()

[spokodev]

Problem

roundTo and round break exact-half ties differently. round rounds half towards +Infinity (mirroring Math.round, as the docs say), but roundTo rounds the magnitude (half away from zero). Since rounding to the nearest multiple of 1 is the same as integer rounding, x.round() and x.roundTo(1) must agree — yet they disagree on every negative half:

new Fraction(-1, 2).round()      //  0
new Fraction(-1, 2).roundTo(1)   // -1   ← differs

new Fraction(-3, 2).round()      // -1
new Fraction(-3, 2).roundTo(1)   // -2   ← differs

new Fraction(-1, 4).roundTo('1/2') // -1/2, but round-to-nearest gives 0

Non-tie values and all positive values were already correct; only negative exact halves were affected.

Cause

roundTo computes k = floor(n / d) on the unsigned magnitudes and bumps k on a tie with if (r + r >= d) k++, then re-applies the sign. Because the half is resolved on the magnitude, ties always round away from zero.

Fix

Use the same sign-aware tie-break that round already uses — round a half up only when the value is non-negative — so ties always break towards +Infinity regardless of the multiple:

if ((this['s'] >= C_ZERO ? C_ONE : C_ZERO) + r + r > d) {
  k++;
}

Tests

Added negative-tie cases to tests/fraction.test.js (-1/2, -3/2 to a multiple of 1; -1/4 to a multiple of 1/2) — red before, green after. Full suite green (317 passing). dist/ rebuilt to match (only the touched logic; no unrelated build churn).