Add int32 fast lane to the raw-double remainder sites

[lahma]

Summary

Three raw-double lanes compute JS % as double % double — a native fmod call — even for the dominant integral loop-counter shapes, while the shared Remainder helper already has an integer fast path for boxed Integer operands:

• JintAssignmentExpression.TryCompoundUnboxed, the x %= y arm
• SimpleAssignmentExpression.TryAssignNumeric, the lhs = a % b arm
• JintBinaryExpression.ModuloEqualityLane, the fused x % c === c stopwatch if-chain shape

Route all three through a shared RemainderUnboxed(double, double) helper: an int32 round-trip gate (NaN, ±Infinity, fractional values, out-of-int32-range operands and zero divisors all fail the round-trip equality and defer to the IEEE 754 %, which matches the ECMAScript algorithm for those cases), remainder computed in long math (int.MinValue % -1 would raise a hardware overflow in 32-bit division), and a zero remainder takes the dividend's sign bit — so negative and -0 dividends produce the spec -0.

Benchmarks (default jobs, adjacent A/B on latest main)

Benchmark	main	PR	Δ
Stopwatch Execute	97.03 ms	92.33 ms	−4.8%
Stopwatch Execute_ParsedScript	96.83 ms	92.02 ms	−5.0%
Stopwatch Execute (Modern)	90.76 ms	88.93 ms	−2.0%
Stopwatch Execute_ParsedScript (Modern)	91.55 ms	88.09 ms	−3.8%
SunSpider crypto-md5 / math-cordic / string-fasta	53.76 / 106.33 / 70.70 ms	54.02 / 107.13 / 71.53 ms	neutral (within drift)

Allocations unchanged on every row.

Correctness

New IntegerRemainderLanesPreserveSignAndSpecialCases test: 63 assertions × 3 loop passes (cache-armed lanes) across all three shapes — dividend-sign results, Object.is(x, -0) for negative and -0 dividends, % 0 → NaN, fractional fallback, int.MinValue % -1 → -0.

Gates

• Jint.Tests: 3542 (net10.0) + 3479 (net472) passed, 0 failed
• Jint.Tests.PublicInterface / CommonScripts: green both TFMs
• Test262: 99,429 passed, 0 failed (an earlier run under heavy CPU contention flaked 8 known slow RegExp-literal tests at their 30 s timeout; clean re-run on an idle machine)

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