stillwater-sc / universal / 35483994330

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feat(ereal): exact digit oracles for multi-thousand-digit ereal (#1573)

Part 4 of #1355. A precision claim can only be checked by an oracle that
carries more digits than the claim: with long double limbs ereal reaches
thousands (#1564), and nothing that passes through a double -- or through
to_string, which rounds -- can see them.

- ereal_reference_digits.hpp: ereal_to_dyadic reads an expansion's limbs
  into an exact dyadic rational (dyadic::from_fp is exact for every limb
  type), and agreed_decimal_digits compares against a decimal reference,
  or against another ereal of any limb type and width.
- The dyadic-only comparators move from elreal_reference_digits.hpp into
  dyadic_exact.hpp, next to the dyadic they measure, so both number
  systems use one implementation. elreal keeps its ZBCL wrappers.
- long_reference_constants.hpp: 9 constants at 5000 digits, emitted by
  generate_reference_constants.py --long. Separate from the 340-digit
  header, which cannot certify a value carrying thousands, and not
  included by any number system: it is 46 KB of string literals.

elastic/ereal/arithmetic/precision_digits.cpp checks the oracle itself
(an exact value scores the cap, a wrong digit is found where it is), the
identities (1/b)*b == 1 and a cancellation per limb type, and sqrt(2) by
Newton iteration -- arithmetic only, no math library -- against the long
reference. ereal<19> reaches 320 digits; ereal<24, long double> reaches
1235, which is the point of the wider limb.

The long double cases run at levels 2 and 3, not in CI's level 1: a
single long double division costs seconds today, because ereal does not
cap a result at maxlimbs and with a wide-exponent limb the expansion
grows to ~250 limbs whatever maxlimbs says (#1572, filed with the
measurements).

Relates to #1355
Resolves #1566

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

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