v3.5.3 — FB calculator loop extraction, STL algorithm improvements

Code quality patch. No public API changes. 37/37 tests pass.

Changed

• ForwardBackwardCalculator pairwise/max-reduce extraction
  (src/calculators/forward_backward_calculator.cpp): the pointer-setup
  block and outer time-loop were duplicated between the Pairwise and
  MaxReduce forward and backward passes. Extracted compute_forward<Fn>
  and compute_backward<Fn> anonymous-namespace templates; the four
  concrete methods are now single-call wrappers. Completes the Tier 2
  plan item that was previously marked done but only partially executed.

• BasicMatrix3D constructor delegation
  (include/libhmm/linalg/basic_matrix3d.h): the zero-fill constructor
  now delegates to the init-value constructor in one line, removing
  duplicated dimension-validation and allocation code.

• STL algorithm improvements (10 distribution files +
  src/common/weighted_stats.cpp): replaces unconditional weight
  accumulator loops with std::accumulate, enabling const on the
  result. Replaces the Weibull data-validation loop with std::any_of.
  Adds explicit #include <numeric> where previously relying on
  transitive inclusion.