stillwater-sc / universal / 25990604877

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feat: exact d2b-based decimal parse via distillation for dd, qd, floatcascade (#851)

* feat(utility, dd, qd, floatcascade): exact d2b-based decimal parse via distillation (Resolves #848)

Replaces the legacy `r *= pown(10, e)` decimal-exponent step in dd::parse,
qd::parse, and floatcascade<N>::parse with an exact route through
decimal_to_binary, distilled into N IEEE-754 doubles forming a canonical
non-overlapping cascade expansion. This closes the residual ULP-level
error the pown path accumulated for large |e|.

New utility (`include/sw/universal/utility/decimal_to_binary.hpp`):
- `distill<N>(result, double[N])`: takes a d2b conversion result and
  produces N doubles in canonical cascade form. Internally:
    * Tracks a SIGNED bigint residual (allows round-up to flip sign,
      with the next component compensating)
    * For each iteration: find MSB, extract top 53 bits with
      round-to-nearest-even (using bigint sticky), convert to double
      via std::ldexp, subtract back exactly from residual
    * |out[i+1]| <= ulp(out[i]) / 2 by construction
- Caller chooses target_mantissa_bits when calling convert() to give
  distill enough explicit bigint precision (53*N + 20 is sufficient).

Migrations:
- dd::parse: digit-accumulate + pown loop -> d2b::convert(payload, 126)
  -> distill<2> -> dd(hi, lo). nan/inf token routing preserved.
- qd::parse: same shape, distill<4> -> qd(x0, x1, x2, x3).
- floatcascade<N>::parse: shared by dd_cascade / td_cascade / qd_cascade.
  d2b::convert(payload, 53*N + 20) -> distill<N> -> assign back to *this.

Tests:
- New `static/utility/test_distill.cpp` exercises the distill helper
  directly: exact-in-double inputs produce single component + zeros,
  0.1 and pi cascades, canonical-cascade invariant over many inputs,
  stod oracle agreement for the hi component.
- dd/qd/cascade existing string_parse tests still pass.
- New large-|e| test group in dd/conversion/string_parse.cpp verifies
  the hi component bit-exactly m... (continued)

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