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feat(elreal): lazy API hardening -- memoisation test + precision-honest approx (#1079 Phase 2) (#1081)

Phase 2 of the elreal class facade.

Adds elastic/elreal/api/lazy.cpp, which validates the lazy state-machine API at
the class-facade level:
- MEMOISATION/REUSE: wraps a counter-instrumented ZBCL in an elreal (via the
  stream ctor) and asserts each tail thunk fires AT MOST ONCE across
  limbs()/approx()/refine() -- refining never recomputes earlier limbs (the
  incremental-precision edge; mirrors zbcl/laziness.cpp at the class level).
- prefix stability (limbs(d) is a prefix of limbs(d')), approx convergence,
  refine()/precision() semantics (refine only raises), stream() round-trip, and
  zero/empty edge cases (limbs(0), empty stream).

Hardens elreal::approx<T>: it now sums IN T (was always double), so
approx<long double> keeps a wider host's extra range/precision instead of
rounding through double; static_assert T is floating-point. The double/float/
long double conversion operators route through approx for one code path. (The
ZBCL-level to_double_approx helper is unchanged.)

el_api_lazy + el_api_api PASS on gcc + clang (Release) and gcc Debug with
assertions on; cppcheck clean; ASCII clean.

Relates to #1079

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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