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feat(interval): Stage 2 EFT-based tight enclosures for native floats (#1247) (#1248)

* feat(interval): Stage 2 EFT-based tight enclosures for native floats (#1247)

Widen an interval endpoint only when an operation is actually inexact, using
error-free transformations (Knuth TwoSum for +/-, Dekker-FMA TwoProduct for *,
FMA residual for the reciprocal in /), recovering 1-ulp-optimal enclosures.
Exactly-representable operations now stay exact (interval<double>(2)+
interval<double>(3) is [5,5], not [nextafter(5,-inf), nextafter(5,+inf)]) while
containment (the Fundamental Theorem, #1234) is preserved.

Safety: the tight path is gated by interval_detail::interval_eft_exact, enabled
for native std::is_floating_point types ONLY. The EFT roundoff term is provably
representable only for an IEEE-754-style format with round-to-nearest AND gradual
underflow. The Universal fixed-size types do NOT qualify and keep the proven
Stage-1 unconditional outward rounding:
  - cfloat without subnormals (the default) flushes the TwoSum error to zero,
    silently breaking containment -- measured 132 breaks / 200k pairs for
    cfloat<16,5>.
  - posit's tapered precision does not guarantee the roundoff is representable.
The trait is safe-by-default: any unrecognized type resolves false and falls
back to Stage-1, so containment can never be silently lost.

Also fixes a sign bug in the EFT reciprocal for negative-denominator intervals:
the widen direction depends on sign(residual)*sign(denominator), not the
residual sign alone.

Tests:
  - addition/subtraction/multiplication/division: exact-operand cases assert
    EXACT equality for EFT types and containment for Stage-1 types, via a
    trait-aware enclosureOK helper.
  - containment.cpp: division added to the fuzz (positive + negative
    denominators, zero-straddling skipped); new Stage-1-fallback containment
    fuzz for cfloat<16,5> and posit<32,2> pinning that they stay sound; the
    tightness bound tightened from 8 ul... (continued)

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