Core operator-splitting solver: ODE + IMEX

[jc-macdonald]

Port the core solver from ACCIDDA/op_engine to Julia.

Scope (package only — no manuscript content)

• Define Problem type wrapping the operator-partitioned system from OpSystem.jl
• Implement explicit ODE integration via OrdinaryDiffEq.jl
• Implement IMEX splitting (implicit diffusion, explicit reaction)
• Implement PDE operator splitting with finite-volume discretisation
• Uniform grid management on arbitrary axes (trait, spatial, etc.)
• Callback interface for diagnostics and output
• Integration tests against Python op_engine reference solutions

Boundary

This issue covers the solver, not the system specification (OpSystem.jl) or the evaluation framework (ModelCriticism.jl). The solver takes a compiled system and returns trajectories.