Dynamic axis addition: add/remove structured axes at specification time

[jc-macdonald]

Support runtime addition and removal of axes in a compiled system specification.

Motivation

Users should be able to extend a model with new trait, spatial, or structured dimensions declaratively. For example, a TRIDENT-like model starts with trait axes (phytoplankton size, zooplankton size) and a spatial axis (depth). Adding a new axis (e.g. light-harvesting pigment ratio) should:

1. Extend the state tensor dimensions
2. Wire diffusion operators for the new axis
3. Adjust reaction terms that depend on the new trait
4. Validate that rate functions have the right arity

Depends on

• Add Axis pydantic model #23 (Axis pydantic model)
• Add StateAxes Pydantic model #27 (StateAxes pydantic model)

Cross-references

• jcm-sci/OpSystem.jl Make SystemABC provider package for flepimop2 #2 — Julia parallel of this issue
• jcm-sci/trident — first consumer (3 trait axes + 1 spatial axis)

Tasks

• add_axis(spec, axis) method on compiled specification
• remove_axis(spec, name) — project out / marginalise a dimension
• Validate reaction term arity after axis changes
• Tensor-product grid assembly: full state = outer product of all axis grids
• Resolution independence per axis