Built-ins are ordinary ModelSpecification values: flowering stem, branching
herb, bush, conifer, broad-canopy tree, vine, compound leaf, raceme, sympodial
flowering plant, phyllotactic head, root-like structure, botanical arch,
botanical hut, and timed tree.
Use PresetRegistry.extend(parentId, childId, overrides) to derive immutable
variants. Presets demonstrate mechanisms; they are not copied species models or
hardcoded generator exceptions.
The six gallery presets deliberately use different architectural programs:
flowering-stem keeps a monopodial leader, branching-herb replaces its apex
sympodially, bush starts from six basal axes, conifer emits tiered whorls,
broad-canopy-tree raises a clear bole before dividing its crown, and vine
alternates a curved leader with tendrils.