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feat(bisection): template AuxReal for high-precision interval bisection (#703)

* feat(bisection): template AuxReal for high-precision interval bisection

Parameterize bisection<G, R, nbits, bt, AuxReal = double> so the
interval bisection pipeline (encode/decode/bisection_point and all
generator/refinement functors) runs entirely in the AuxReal type.

For nbits up to ~50, double (the default) provides sufficient
precision for the binary search to converge to the correct encoding.
For wider encodings, instantiating with dd (~31 digits) or qd (~63
digits) from the Universal library keeps the pipeline accurate.
This is the approach recommended in Section 4 of the paper.

Changes:
- bisection_fwd.hpp: add 5th template parameter AuxReal (default double)
- bisection_impl.hpp: bisection_point, bisection_encode, bisection_decode
  templatized on <R>; class gains decode_aux()/assign_from_aux() helpers;
  arithmetic operators route through AuxReal instead of hardcoded double
- generators.hpp: all generator/refinement operator() become function
  templates; use ADL-qualified math calls (using std::log2/exp2/...) so
  dd/qd overloads are found via argument-dependent lookup
- numeric_limits.hpp, manipulators.hpp, attributes.hpp: thread AuxReal
- Type aliases gain an optional AuxReal parameter (default double)
- New test: static/bisection/conversion/aux_real.cpp exercises dd- and
  qd-backed encode/decode round-trips and monotonicity at 8 and 16 bits
- tools/bisection REPL: register bisection_posit{16,32}_dd and
  bisection_natposit16_dd for interactive exploration

Storage remains int64_t (nbits <= 64). Promoting to blockbinary for
wider formats is tracked as a follow-up under Epic #687.

Resolves #692
Relates to #687

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(bisection): correct comment: 1/7 is rational, not irrational

The comment in aux_real.cpp incorrectly called 1/7 "an irrational
fraction". 1/7 is rational; the relevant prop... (continued)

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