fix double rounding of hex-floats in StringToFloat - #304
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Repro: StringToFloat("0x8a4.d047p-140") returns 1.587615108e-39 where strtof and the exact value round to the neighbouring float 1.587616509e-39. Hex-floats landing in the subnormal range, or whose exact significand exceeds 53 bits (e.g. "0x100000100000008p0"), can be a ULP out.
Cause: RadixStringToIeee rounds the significand to single precision for the float path, and StringToFloat then casts the resulting double to float, so the value is rounded twice. The decimal path already avoids this double rounding; the hex-float path did not.
Fix: keep the full double significand for hex-floats and round it to odd, leaving the single rounding to the caller's cast. Regression tests for the subnormal and >53-bit cases sit beside the existing hex-float tests.
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Repro: StringToFloat("0x8a4.d047p-140") returns 1.587615108e-39, where strtof and the exact value round to the neighbouring float 1.587616509e-39. Hex-floats landing in the subnormal range, or whose exact significand exceeds 53 bits (e.g. "0x100000100000008p0"), can be a ULP out.
Cause: RadixStringToIeee rounds the significand to single precision for the float path, and StringToFloat then casts the resulting double to float, so the value is rounded twice. The decimal path already avoids this double rounding; the hex-float path did not.
Fix: keep the full double significand for hex-floats and round it to odd, leaving the single rounding to the caller's cast. Regression tests for the subnormal and >53-bit cases sit beside the existing hex-float tests.