stillwater-sc / universal / 25973636098

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feat(posit): route decimal string parse through decimal_to_binary (Phase B2b of #835) (#843)

* feat(posit): route decimal string parse through decimal_to_binary (Phase B2b of #835)

Replaces the std::istringstream double funnel in posit::parse() with a direct
call into the high-precision decimal_to_binary utility shipped in Phase B2a.
For posit configurations whose fraction precision exceeds double's 52 bits
(roughly posit<nbits, *> with nbits > 60), this captures the trailing decimal
precision that the legacy path silently truncated.

Implementation:
- Add #include <universal/utility/decimal_to_binary.hpp>
- Decimal branch of parse() now:
  * Routes "nan" / "inf" / "infinity" tokens (any case, optional sign) to NaR
  * Calls decimal_to_binary::convert with target_mantissa_bits = nbits + 5
  * Copies nbits+4 mantissa bits into a blocksignificand<extractBits>, folds
    the d2b residual guard/sticky into bit[0], and hands off to convert_<>()
    so all rounding happens once in posit's encoding step
- Hex format path (regex match on nbits.esxHEXVALUEp) is unchanged

Tests (static/tapered/posit/conversion/string_parse.cpp):
- posit<32,2>: bit-exact match against posit(double) for canonical inputs
- posit<16,2>: basic decimals
- posit<8,0>: native hex format round-trip, mismatched nbits.es rejected
- posit<32,2>: nan/inf literals (10 spellings) -> NaR
- posit<32,2>: operator>> bad-token handling
- posit<64,3>: exact-in-double inputs round identically; 1e-6 diverges (expected:
  posit<64,3> has ~56 fraction bits, more than double's 52, so the new path
  captures precision the funnel lost)
- posit<128,4>: wide decimal of pi parses to a defined non-zero non-NaR value

Verified on gcc (build_ci) and clang (build_ci_clang): posit_string_parse,
posit_api, posit_assignment, posit_conversion all PASS.

Relates to #835

* fix(posit): address CodeRabbit round-1 review on PR #843

Three actionable items from CR review #1:

1. Add direct <cctype> and <string_view> i... (continued)

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