stillwater-sc / universal / 25320913136

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fix(qd): to_digits robust to non-canonical limbs (qd/dd/floatcascade) (#808)

* fix(qd,dd,floatcascade): make to_digits robust to non-canonical inputs

to_digits()'s iterative digit extraction assumes the operand is in
canonical limb form (high-magnitude-first, with the limb-overlap invariant
|x[i+1]| <= ulp(x[i])/2).  Non-canonical inputs -- easily constructed via
the public raw-limb constructors qd(double, double, double, double) /
dd(double, double) / cascade equivalents -- drove the loop to NaN, which
surfaced as either a stderr warning ("to_digits() failed to compute
exponent", "to_digits() non-positive leading digit") or, before PR #800's
defensive guard, as C++20 [conv.fpint] UB at static_cast<int>(r[0]).

Reproduction (issue #801):
    qd a(1.0, 0.0, 0.0, std::pow(2.0, -196.0));
    std::cout << a << '\n';   // pre-fix: stderr warning, garbage output

Fix: call renormalize() at the start of to_digits() so the algorithm
operates on a canonical copy regardless of caller invariants.  The cost
is one renormalization per to_string call (which already does many high-
precision operations), so the perf impact is negligible.

Changes
-------

include/sw/universal/number/qd/qd_impl.hpp:
  * to_digits(): call r.renorm() after `qd r = abs(*this)`.
  * Reverted PR #800's defensive NaN guard at the static_cast<int>(r[0])
    site (commit ac093fce); the renorm above keeps r canonical at entry,
    so the algorithm holds r[0] in [0, 10) at each iteration without the
    backstop.  Restores the original simpler digit-extraction loop.

include/sw/universal/number/dd/dd_impl.hpp:
  * Added dd::renorm() member (mirrors qd::renorm; uses quick_two_sum
    on (hi, lo) to re-establish the canonical pair).
  * Guarded with an inf-check at the maxpos boundary: when hi is at
    DBL_MAX and lo is positive, hi + lo overflows to +inf and
    quick_two_sum corrupts both limbs (s = inf, lo = -inf).  In that
    case the dd is canonical by construction and skipping renorm i... (continued)

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