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feat(efloat): implement mathematical truncation and rounding library (#1125)

* feat(efloat): implement mathematical truncation and rounding library (#1112)

Implement trunc(), floor(), ceil(), round(), rint(), nearbyint(), lrint(), and llrint()
inside math/truncate.hpp, performing standard mathematical rounding completely at the
exact bit-level of efloat's own normalized representation.

Declare these mathematical functions as friend functions inside efloat_impl.hpp,
granting them direct, high-performance permission to clear fractional bits of the
protected _limb vector.

Register math/truncate.hpp inside our master efloat mathlib.hpp header.

Create truncate.cpp inside elastic/efloat/math/ to verify all truncation and rounding
functions across all positive, negative, and exact halfway-tie boundary states.

Resolves #1112
Relates to parent Issue #1092, Epic #1101

* fix(efloat): resolve CodeRabbit review comments on truncation library

1. Implement high-precision, lossless lrint() and llrint() inside math/truncate.hpp:
   convert the rounded efloat directly at the bit-level without double-cast, preventing
   precision truncation for values above 2^53.

2. Preserve negative zero: save the original sign before clear_fractional_limbs() and restore
   it on res.iszero() under trunc, floor, ceil, round, and rint, matching standard IEEE-754
   and cmath sign preservation.

3. Implement rounding-mode rint(): update rint() to branch on the active efloat_rounding_mode
   under RoundToNearest (ties-to-even), RoundToZero (truncation), RoundTowardPositive,
   and RoundTowardNegative, matching std::rint behavior.

4. Fix sticky-bit boundary check: correct the first_extra_limb starting index inside round()
   and rint() to prevent skipping full_limbs + 1 when bit_idx < 31, successfully
   matching all sticky bit transitions.

5. Update regressions: expand truncate.cpp to cover rint() dynamic rounding mode selections,
   lossless llrint() above 2^53, nearbyint() re... (continued)

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