Avoid Float out of range warning for clearly underflowing numbers

[znz]

Two new minefield parser tests (test_i_number_double_huge_neg_exp, test_i_number_real_underflow) introduced in 6507a836c5 cause spurious CI warnings because numbers like 123.456e-789 and 123e-10000000 fall through to json_decode_large_float → rb_cstr_to_dbl → strtod, which sets errno=ERANGE on underflow and triggers Ruby's rb_warning("Float %s out of range", ...).

Fix

Add an early-return guard in json_decode_float() before the rb_cstr_to_dbl fallback:

if (RB_UNLIKELY(mantissa_digits > 18 || mantissa_digits + exponent < -307)) {
    if (RB_UNLIKELY(mantissa_digits + exponent < -324)) {
        return rb_float_new(negative ? -0.0 : 0.0);
    }
    return json_decode_large_float(start, end - start);
}

When mantissa_digits + exponent < -324, the value is bounded above by 10^(-324) = 1e-324 < DBL_TRUE_MIN/2 ≈ 2.47e-324, so IEEE 754 round-to-nearest guarantees a result of 0.0 — no need to call rb_cstr_to_dbl. The subnormal range (-324 ≤ mantissa_digits + exponent < -307) still goes through rb_cstr_to_dbl for accurate results.

Same approach as 5b4d95b8d0 which added the exponent < INT32_MIN → 0.0 fast-path with the same motivation.