lib: fix evenRound sign for fractional values in (-1, 1)

[deepview-autofix]

evenRound derived the rounding direction from MathSign(i) where i = integerPart(x). For any x whose truncated integer part is zero (e.g. 0.7, -0.7), sign was 0, so the non-halfway branch returned i + sign === 0 instead of rounding away from zero.

This produced incorrect [Clamp] WebIDL integer conversions for fractional inputs in (-1, 0) ∪ (0, 1) that are not exactly ±0.5

Observable e.g. via new Blob([Uint8Array.of(1,2,3)]).slice(-0.9999999) returning the whole blob instead of a 1-byte slice.

Use MathSign(x) so the rounding direction follows the sign of the original value.

[ChALkeR]

This is mine (scanner + autofixer)

---

Node.js, prior to the patch:

---

Chrome:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.62%. Comparing base (14e16db) to head (cc729e8).
⚠️ Report is 112 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #62837 +/- ##
=======================================
Coverage 89.62% 89.62% =======================================
Files 706 706 Lines 219136 219136 Branches 41987 41981 -6 =======================================
+ Hits 196404 196410 +6 - Misses 14611 14614 +3 + Partials 8121 8112 -9 

Files with missing lines	Coverage Δ
lib/internal/webidl.js	96.62% <100.00%> (ø)

... and 26 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[ChALkeR]

Closing in favor of #62979