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feat(ereal): precision-lifting self-oracle (#954 D2) (#984)

* feat(ereal): precision-lifting self-oracle (#954 D2)

Deliverable 2 of #954: a self-referential precision-lifting oracle that uses
the widest configuration ereal<19> (1007 bits) as the value-canonical ground
truth for narrower ones -- no external oracle.

ereal_test_support.hpp:
- widen<WIDE>(ereal<NARROW>): value-preserving lift of a narrow expansion into
  a wider configuration by re-summing its non-overlapping limbs.

elastic/ereal/arithmetic/precision_lifting.cpp:
- For config pairs (2,19), (4,19), (8,19), (16,19): generate narrow inputs,
  lift them to ereal<19>, and assert widen(a_n op b_n) == (a_w op b_w) for
  +, -, *, / over random multi-component expansions (LEVEL_1..4 = x1k..x1M).

The comparison is EXACT (not "up to truncation"): ereal operations are
maxlimbs-independent -- maxlimbs is the algorithmic ceiling on inputs, not a
runtime truncation, so a narrow result carries the same value as the wide one.
A probe over 5,000 cases confirmed bit-identical narrow-vs-wide results for all
four operators; the fuzzer found 0 failures through x100k on gcc-13 and
clang-18, so no arithmetic operator needs a skip-list entry.

(This oracle depends on #981 having made * and / canonical.)

Relates to #944
Relates to #954

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* fix(ereal): static_assert WIDE>=NARROW in widen() (CodeRabbit PR #984)

Enforce the widening contract of the precision-lifting helper at compile time.

Relates to #954

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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