stillwater-sc / universal / 29688711546

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feat(efloat): implement decimal operator<< / to_string (was a 'TBD' stub) (#1150) (#1155)

* feat(efloat): implement decimal operator<< / to_string (was a 'TBD' stub) (#1150)

efloat's stream output printed the literal "TBD" for every finite value; only
inf/nan/zero worked. Implemented an arbitrary-precision binary->decimal formatter
(to_string + to_digits), ported from ereal: digits are extracted using efloat's
own arithmetic (scale to [1,10) via *10 / /10, then repeatedly take the leading
digit and multiply by 10), with round-to-nearest and carry propagation. Honors
the stream's precision/width and fixed/scientific/showpos/uppercase/internal/
left flags; operator<< now delegates to it (same structure as ereal's).

Sign uses sign()==-1, not isneg() (which is false for -inf), so -inf prints
correctly. Defined after the arithmetic operators it uses (forward-declared
before operator<<). Accuracy is bounded by the operand's working precision.

New elastic/efloat/conversion/to_string.cpp: no-TBD, std match for double-range
scientific values, high-precision round-trip (pi to 20/50/100 digits vs the
efloat_pi oracle), finite round-trips, fixed format, and special values (incl.
the -inf sign). Validated on gcc 13.3.0 and clang 18.1.3.

Resolves #1150

* fix(efloat): decimal output -- large exponents + fixed-format rounding (CodeRabbit #1155)

Two porting bugs in the new to_string:
- append_exponent used ereal's fixed-width e/100 digit math, which emits
  non-digit chars for |exponent| >= 1000. efloat's range far exceeds double's
  (it can print 1e1000), so switch to a generic std::to_string path with
  minimum-2-digit padding. (also resolves the Codacy static-analysis finding)
- the fixed path called round_string(t, nrDigitsForFixedFormat + 1, ...) and
  copied nrDigitsForFixedFormat digits, so it read the NUL terminator (never
  rounded) and emitted too many decimals for values < 1. Use nrDigits (the
  requested integerDigits + precision) as ereal does; the... (continued)

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