Stage Size - configurable NxN grid (2×2 / 3×3 / 4×4)
Add a Stage Size selector to the Object Collection panel (addon v0_49, generator v0_28). The user picks from three grid layouts before creating Generation 1: 2×2 Classic (4 stages), 3×3 Default (9 stages, the standard layout), or 4×4 Testing (16 stages). The hardcoded 3×3 stages list in the generator is replaced by a build_stage_grid(n) function that computes positions dynamically. Each stage keeps the same physical size; only the grid footprint changes. The grid size is stored in the scene property gaad_grid_size and recorded in the generation JSON (world.stage_grid, world.stage_count). The selector is hidden once Gen 1 exists (same visibility rule as the primitive toggles).