stillwater-sc / universal / 26972513907

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feat(elreal): lift the 2^-1022 precision floor to reach 300+ digits (#1052) (#1055)

* feat(elreal): lift the 2^-1022 precision floor to reach 300+ digits (#1052)

The elreal constant generators plateaued at ~276-282 decimal digits regardless of
depth -- short of the ~308-digit architectural ceiling of a double host -- because
the arithmetic pipeline floored at the double normal range (~2^-1022) instead of
the block's symbolic int32 exponent. The exact-dyadic oracle (#1048) measured the
plateau; this lifts it. Diagnosis (two layered causes, each verified with a spike):

1) div() floor. divide.hpp stopped the long division at
   exp_floor = min_exponent + 2k (~2^-915 for double). But the block EFTs
   (block_two_mult / block_two_div) are scale-invariant -- they operate on the
   normalised significand v in [1,2) (always a normal host value) and track the
   scale symbolically in exp -- so the remainder reductions never denormalise on a
   wide host. Lifting the floor lets div() reach its true ~19-component / ~308-digit
   ceiling (measured: div(1/7) 276 -> 308 digits). Gated to wide hosts (k>=24);
   narrow hosts (bfloat16 k=8, fp16 k=11) genuinely denormalise near their small
   min_exponent and keep the floor (the same one #1044 respects).

2) series-accumulation loss. Even with div() lifted, pi/ln2/e capped ~2 components
   below div(): odd_power_series / e_zbcl ran their internal mul/div/sum at the
   OUTPUT depth, so accumulation error consumed the deepest output blocks. They now
   carry kSeriesGuard (=4) extra working blocks internally (output + guard) and use
   the wide-host series_stop_exp(), absorbing the loss.

Result (depth 20, vs the 320-digit reference_constants.hpp):
  pi 307, e 309, ln2 308, ln10 308, log2_10 309, phi 312, sqrt2 311, sqrt3 308,
  sqrt5 312 -- all >= 300 (was 276-282). constants_highprecision.cpp threshold
  raised 270 -> 300. euler_gamma stays a stub (#1053).

Ceiling note: ~308-312 digits is the hard double-host limit... (continued)

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