Dynamic axis management: add/remove/reshape axes at specification time

[jc-macdonald]

Support declarative addition and removal of axes in the system specification, so users can straightforwardly extend a model with new trait, spatial, or structured dimensions.

Motivation

A TRIDENT-like model starts with trait axes (phytoplankton size θ, zooplankton size ϑ) and a spatial axis (depth z). A user should be able to:

1. Add a new trait axis (e.g. light-harvesting pigment ratio) with one YAML block
2. The compiler should automatically:
   • Extend the state tensor dimensions
   • Wire diffusion operators for the new axis
   • Adjust reaction terms that depend on the new trait
   • Validate that rate functions have the right arity

Tasks

• Axis type hierarchy: TraitAxis, SpatialAxis, StructuredAxis (age, stage, etc.)
• Each axis declares: name, bounds, default resolution, diffusion coefficient (optional), boundary conditions
• add_axis!(spec, axis) — extend a compiled SystemSpec with a new dimension
• remove_axis!(spec, name) — project out a dimension (integrate or marginalise)
• Validate: reaction terms must accept the correct number of trait arguments after axis changes
• Tensor-product grid assembly: the full state is the outer product of all axis grids
• Resolution independence: axes can have different resolutions; the compiler handles indexing

Cross-references

• OpEngine.jl Dynamic axis management: add/remove/reshape axes at specification time #2: the adaptive grid refinement consumes axis declarations from here
• OpEngine.jl Core YAML-to-callable system compiler #1: the solver must handle variable-dimension state vectors
• trident Core YAML-to-callable system compiler #1: TRIDENT is the first consumer — 3 trait axes + 1 spatial axis
• ACCIDDA/op_system: Python equivalent should get a parallel issue

Boundary

This is specification — how axes are declared and validated. The grid points (uniform or adaptive) are computed by OpEngine.jl. The evaluation of whether adding an axis improves predictions is ModelCriticism.jl.