stillwater-sc / universal

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fix(posit): correct NaR in to_binary, to_triple, and color_print (#561)

Three related bugs in posit display functions:

1. extract_fields() did not handle the NaR bit pattern (sign bit set,
   all other bits zero). Two's complement of the minimum signed integer
   wraps to itself, causing decode_regime() to produce wrong regime bits.
   Add NaR detection to extract_fields() so it properly sets regime to
   inf (all zeros consuming nbits-1) with empty exponent and fraction.
   This fixes to_binary() and to_triple().

2. color_print() inverted decoded field bits for negative posits trying
   to reconstruct the raw encoding, but bit inversion is not two's
   complement (missing +1), producing wrong bit values. Fix: read the
   decoded field bits directly without inversion, matching to_binary().

3. Exponent and fraction to_string() emitted nibble separator characters
   even when no content bits were output (dashExtent=false with zero
   allocated bits), producing spurious trailing apostrophes for NaR and
   zero in larger posit sizes.

Resolves #559

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

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