stillwater-sc / universal / 34995207204

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fix(areal): convert to long double with ldexp in the target type (#1512)

Converting an areal to long double gave wrong values (#1509). On clang,
long double(areal<32,8>(0.75)) was -nan and 1.0/3 came back as twice the value.
On every compiler a value outside double's exponent range came back as inf or 0,
and so did every subnormal of a configuration with es >= 12.

to_native() built the power of two three ways, each with its own defect:

- inside (-64, 64) from 1ull << e. clang 18 miscompiles
  1 / (long double)(1ull << n) for a runtime n, at -O0 and -O2 alike. A signed
  shift and a plain uint64_t argument both convert correctly. It is the fault
  #937 worked around in cfloat.
- outside that range as a double from ipow(), which caps a long double at
  double's range. ipow also computed 2^-e as 1 / 2^e, so for e <= -1024 it
  divided by inf and flushed double subnormals to 0 as well.
- for subnormals from subnormal_exponent[es], a table of doubles that holds 0
  for es >= 12.

All three are now std::ldexp in TargetFloat, and the ipow() helper is gone.

static/range/areal/conversion/native_conversion.cpp checks against a reference
that decodes an encoding's fields with integer arithmetic and std::ldexp,
without any areal conversion. It checks every encoding of the small
configurations. For areal<32,8>, <64,11>, <64,12> and <64,15> it checks every
exponent field, with limbs straddled, against float, double and long double
wherever the target holds the significand exactly. It also round-trips
long double -> areal<128,15> -> long double. denorm_min is included only where
the areal holds it: a quad long double's 2^-16494 is one binade below
areal<128,15>'s minpos. Against the old header it fails on gcc and on clang.
Verified on x86 with gcc and clang, and under qemu-riscv64 and qemu-aarch64
(113-bit long double).

BEHAVIOUR-CHANGE: long double(areal) is now correct on clang (was nan or scaled by a power of two) and holds values outside double's range; dou... (continued)

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