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feat(elreal): Phase 4 -- bit-perfect McCleeary threeAdd + add() (#940)

* refactor(elreal): block exp_offset (coarse) -> exp (per-unit)

Foundational refactor for the McCleeary algorithm work in Phase 4.

The original block<FpType> stored exp_offset as a coarse multiplier in
units of 2^E_FpType (~2048 for double). The intent was to extend a single
block's exponent range, but McCleeary's algorithm (threeAdd, addRec,
createZero) needs per-unit integer exponents in Z.

Changes
- block.hpp: rename `exp_offset` -> `exp`, drop the `* exp_step`
  multiplication in exponent(). For zero blocks exp is returned
  directly (zero blocks have no v-derived scale).
- block.hpp: add the dissertation primitives needed by Phase 4:
  createZero, singleBit, dominate, expGreater, expGreaterBy. (These
  were always going to land here; folding into this refactor keeps the
  block API in one place.)
- block_eft.hpp, block_manipulators.hpp, zbcl_helpers.hpp, zbcl.hpp:
  updated to use the renamed field.
- exp_field_width.hpp: removed (was only needed for the coarse
  multiplier story).
- elreal.hpp: drop the obsolete include.
- elastic/elreal/block/exp_offset.cpp: rewritten to test the simpler
  per-unit semantics (file name kept for stable cmake target naming).
- All other Phase 1-3 tests: trivial field renames + the one cross-exp
  test in zero_overlap.cpp updated to use gap=k instead of an exp=1
  block that no longer means "a huge magnitude jump".

Validation
- gcc 13.3 debug + release: all 13 Phase 1-3 tests PASS.
- clang 18.1 debug + release: all 13 Phase 1-3 tests PASS.

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

* feat(elreal): Phase 4 -- bit-perfect McCleeary threeAdd + add()

Phase 4 of the McCleeary LFPERA reimplementation (epic #923). Direct
C++ translation of FCL.hs from the dissertation Appendix A.4. The
structure mirrors the Haskell so the code can be cross-checked
line-by-line against the dissertation.

What ships
- twoSumRN (Definiti... (continued)

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