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feat(elreal): sqrt and hypot (Phase 7.2, #931) (#1038)

First math-suite sub-phase of Epic #923 Phase 7, building on Phase 6 mul/div.

- math/sqrt.hpp: sqrt(a, depth) -- Newton-Raphson x_{n+1} = (x_n + a/x_n)/2 on
  the lazy stream. Quadratic convergence; seeded from the host-double sqrt of the
  leading approximation and run with geometrically growing working depth
  (1,2,4,...,depth) so early low-accuracy steps stay cheap. Iterations =
  3 + ceil(log2(depth+1)) (mirrors the ereal precedent). Domain: sqrt(0)=0
  (empty), sqrt(<0)=empty.
- math/hypot.hpp: hypot(x,y,depth) = sqrt(x*x + y*y). Expansion arithmetic carries
  the squares exactly (block exponents are int32_t), so the naive formula is safe.

New elastic/elreal/math/ test directory (el_math CMake group):
- sqrt.cpp: perfect squares exact via the dyadic oracle; irrational roots vs
  std::sqrt and round-trip sqrt(a)^2 ~= a (host-scaled tolerance); 0-overlap;
  domain (sqrt(0)=empty, sqrt(<0)=empty).
- hypot.cpp: Pythagorean triples + general args vs std::hypot; 0-overlap.

Tests parameterised over {double, float, bfloat16}. All 27 elreal tests pass on
gcc and clang (Release) and with assertions on (0-overlap holds on every result).

Relates to #931 (sub-phase 7.2 of the Phase 7 math suite).

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

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