Voronoi-based polycrystal generator

[petlenz]

Opens the metals / alloys market — the largest computational-mechanics
community. Direct competition with Neper (the gold standard, free,
mature, dominant). Per ROADMAP.md this is a Horizon 3 expansion item;
not the headline feature, but it dramatically increases addressable
users.

Proposed fix

template <typename T = double>
class polycrystal_generator final : public rve_generator_base<T> {
public:
  // Schema fields:
  //   target_grain_count: number of grains in the unit cell
  //   grain_size_distribution: name of a distribution (lognormal)
  //   tessellation: "voronoi" | "laguerre" (radius-weighted)
  //   periodic: bool (use periodic Voronoi)

  shape_vector compute(input_vector& inputs,
                       termination_base<T> const& term,
                       std::array<value_type, 3> const& domain_box,
                       progress_options const& opts = {}) override {
    // 1. Sample seed points (count from termination's target).
    // 2. Compute periodic Voronoi (or Laguerre) tessellation.
    // 3. Convert each cell to a `polyhedron_cell<T>` shape.
    // 4. Assign per-grain orientation (uniform random or A2-driven).
    // 5. Return shape vector.
  }
};

Required dependencies

• Voro++ or CGAL for the tessellation. CGAL is heavier but
  produces topologically clean cells with periodic support; Voro++
  is lighter but the periodic implementation is less robust.
• A new polyhedron_cell<T> shape: stores its vertex list and face
  topology, implements is_inside via half-space tests.

Two key challenges

1. Cell-cell collision — for polycrystal generation, cells fully
fill the domain (no inter-cell gaps). The collision check is
unnecessary if generation is via tessellation directly. Implement as
a collision_details overload that returns false when the cells share
a face (which they always do in tessellation output).

2. Periodic boundary handling — for FFT homogenization, the
tessellation must be periodic. Both CGAL and Voro++ have periodic
modes; Neper uses a custom routine.

Output extensions

• gmsh emits Volume(...) per cell with Physical Volume("grain_N")
  per-grain tags.
• DAMASK material.config integration: per-grain orientation +
  material — natural fit for crystal plasticity FFT homogenization.

Acceptance

• polycrystal_generator registered for "type": "voronoi".
• Periodic tessellation correctness: opposite faces match.
• Grain-size statistics control: target lognormal distribution
  with mean and std-dev parameters.
• Test: generate 100 grains in a unit cube with target grain
  size distribution; assert empirical histogram matches the
  input within tolerance.
• DAMASK material.config emission for crystal plasticity.

Out of scope for this PR

• Texture (orientation distribution) — depends on #112 FOD support
  landing.
• Grain growth simulation, recrystallization — material-history
  modelling, not generation.
• Mesh-conformal grain boundaries — use the standard gmsh meshing
  on the tessellation output.

Branch with full draft + design notes: feature/voronoi-polycrystals (docs/issues/114-*.md).