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feat(bposit): the bounded posit, Gustafson's b-posit <N, rS, eS> (#1634)

* feat(bposit): the bounded posit, Gustafson's b-posit <N, rS, eS>

A posit whose regime is capped at rS bits (Gustafson, "Every Bit Counts";
arXiv 2603.01615). The regime is a run that counts its terminating bit, and
a run that reaches rs ends without one, so it takes 2..rs bits, r lies in
[-rs, rs-1], and the exponent is always the full es bits. Every encoding
keeps at least nbits - 1 - rs - es fraction bits; the scale is bounded to
[-rs 2^es, rs 2^es - 1] -- 2^-192 .. 2^192 for the standard rS = 6, eS = 5.

Spec: docs/number-systems/bposit.md (#1251), with the range/precision
profile, numeric_limits, a reference encoding table for bposit<8,4,0>, and
the negative result that bounding the taper does NOT make TwoSum or
TwoProduct exact (exhaustive counts for two 8-bit configurations, and
failures in bposit<16,6,5>).

Type: bposit<nbits, rs, es, bt>, with bposit16/32/64 = bposit<n, 6, 5>.
- Compile-time constraints: 2 <= rs < nbits - 1, nbits > 1 + rs + es (a
  fraction bit at every magnitude), nbits <= 64.
- Posit-standard rounding: nearest, ties to the even encoding; never to 0
  or NaR; clamped to +/-minpos and +/-maxpos. 2^minscale exactly is the
  zero pattern and clamps to minpos.
- A 64-bit integer codec (constexpr); + - * / through blocktriple,
  rounded once; native conversions decompose the double exactly;
  comparison is integer order with NaR below every real.
- Trivially constructible; numeric_limits, type_tag, to_binary
  (sign.regime.exponent.fraction), bposit_range, iostream; layered
  core / manipulators / iostream / attributes headers (#1334).
- UNIVERSAL_BUILD_NUMBER_BPOSITS, in the statics cascade.

Tests (static/tapered/bposit):
- conversion: every encoding of six small configurations and both 16-bit
  ones against an independent bit-string decoder -- value, monotonicity,
  F_min fraction bits, maxpos/minpos formulas, double round trip, every
  midpoint ties-... (continued)

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