Optimize highway Exp using floating-point rounding (round_small technique).

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Optimize highway Exp using floating-point rounding (round_small technique).

Use the magic-constant float rounding trick (as in YNNPACK
round_small) for range reduction: (x * log2(e) + magic) - magic,
where magic = 1.5 * 2^mantissa_bits. This eliminates both the sign check
((x < 0) ? -0.5 : +0.5) and integer conversions (ToInt32 / ConvertTo),
keeping range reduction and exponent scaling entirely within floating-point
and bitwise operations.

Benchmark results on x86_64 (:math_benchmark):

• f32x16: 19.57 - 20.27 ticks (was 27.06 - 41.09 ticks, ~28-50% faster)
• f32x8: 20.01 ticks (was 24.30 - 31.00 ticks, ~18-35% faster)
• f32x4: 17.46 ticks (was 19.39 - 73.74 ticks, ~10-76% faster)
• f32x2: 17.55 - 42.01 ticks (was 80.88 ticks, ~48-78% faster)
• f32 scalar: 16.13 - 18.77 ticks (was 29.96 - 31.16 ticks, ~40-46% faster)
• f64x8: 24.69 ticks (was 31.35 - 34.49 ticks, ~21-28% faster)
• f64x2: 18.63 ticks (was 22.75 ticks, ~18% faster)
• f64 scalar: 17.40 - 20.50 ticks (was 23.34 - 28.92 ticks, ~25-35% faster)