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)

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Node.js, prior to the patch:

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Chrome:

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.62%. Comparing base (14e16db) to head (cc729e8).

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

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