stillwater-sc / universal / 35390754257

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fix: elreal narrow-host convergence and extremes; cfloat ldexp, frexp and saturation (#1559)

#1396: pi_zbcl<half> stopped at 8 digits at every depth. Three stacked causes:
- cfloat ldexp rewrote the exponent field, wrong for every subnormal input
  (ldexp(2^-24, 24) on half gave 1.00098), so block::normalise() corrupted
  subnormal blocks on half. ldexp now keeps the field rewrite only for a
  normal value that stays normal, and otherwise rounds once through
  normalize()/convert(). frexp made the mirror-image error, which the round
  trip hid; it now scales through ldexp.
- the product producers (singleMultHelper, mul, mul_scalar, div) dropped a
  block_two_mult output whose host value was subnormal. It is an exact part of
  the product; it is now normalised and kept (keep_product_block).
- singleDiv_step ended the division at the first empty remainder, dropping
  the rest of the dividend after an exactly divisible prefix. It now reads on,
  skipping a short run of zero blocks, and stops only at the end of the
  dividend or a zero run longer than kSingleDivZeroRun (a lazily produced
  exact value can end in zeros that never stop coming).

#1397: sqrt2_zbcl<half> stalled at 35 digits (the product residual above);
sqrt2 on double scored 319 against a 320-digit reference because that
reference is rounded. The references are regenerated at 340 digits.

#1463: elreal's SpecificValue extremes were double literals on every host;
they are now the host's, matching numeric_limits, which now compiles for
half and bfloat16 hosts.

cfloat: a saturating cfloat returned inf on overflow when rounding landed on
the inf pattern rather than the NaN pattern; convert() now clamps both.

BEHAVIOUR-CHANGE: cfloat ldexp and frexp return correct results for subnormal inputs, and ldexp rounds underflow and overflow
BEHAVIOUR-CHANGE: saturating cfloat arithmetic that overflows gives maxpos/maxneg instead of inf
BEHAVIOUR-CHANGE: elreal products and quotients keep bits they dr... (continued)

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