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feat(elreal): negation, multiplication, division (Phase 6, #930) (#1037)

* feat(elreal): negation, multiplication, division (Phase 6, #930)

Round out the field operations on ZBCL<FpType> (dissertation 4.2.4/4.2.5/4.2.6),
building on the corrected add()/sum()/priestRenorm foundation.

- negate.hpp: negate(x) -- lazy per-block sign flip ({-v, exp}). value(-b) =
  -value(b) exactly; the combined exponent is unchanged by a sign flip, so
  0-overlap is preserved with no renormalisation. empty -> empty.

- multiply.hpp: mul(x, y, depth) -- finite-prefix product. Materialise the first
  `depth` blocks of each operand, multiply out with block_two_mult, and
  renormalise the pooled partial-product blocks with priestRenorm. Exact for
  finite operands; truncates infinite operands at `depth` (full anti-diagonal
  streaming deferred to Phase 7). Also mul_scalar(s, y) (block x ZBCL). Keeps
  only normalised product blocks (drops zeros and sub-floor denormals).

- divide.hpp: div(x, y, depth) -- long division on expansions. The remainder is
  carried as an eager, priestRenorm'd block list (NOT a lazy ZBCL): each step
  takes the next quotient block q = block_two_div_rn(rem0, y0), subtracts q*y
  from the remainder, and renormalises. Refinement stops on exact remainder, the
  depth budget, a no-progress guard, or a margin above the denormal floor
  (min_exponent + 2k) -- denormal blocks have no implicit leading bit and would
  break 0-overlap accounting (hit quickly by bfloat16, k=7). Carrying the
  remainder eagerly avoids a lazy add()-tower and never forces a near-floor
  denormal. Divide-by-zero (empty divisor) returns the empty co-list, or throws
  elreal_divide_by_zero under ELREAL_THROW_ARITHMETIC_EXCEPTION.

- exceptions.hpp: add elreal_divide_by_zero + the ELREAL_THROW_ARITHMETIC_EXCEPTION
  guard (default 0), consistent with the other Universal number systems.

Tests (elastic/elreal/arithmetic/, dyadic-exact oracle, hosts {double,float,bfloat16}):
- negat... (continued)

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