MakieExt and ControlPlotsExt define VortexStepMethod.plot_geometry for the same type

[ufechner7]

This is a bug in VortexStepMethod v3.3.2 — both the MakieExt and ControlPlotsExt define VortexStepMethod.plot_geometry for the same type, and when both Makie and ControlPlots are loaded simultaneously, the second extension can't precompile because it would overwrite the method.

Fix: https://www.perplexity.ai/search/73df42dc-af7c-4d3c-8a8e-5135e0d044a6

[ufechner7]

Fix — three-part change:

VortexStepMethod.jl — Added abstract type PlotBackend end with concrete MakieBackend and ControlPlotsBackend subtypes, a _PLOT_BACKEND ref for tracking the active backend, a set_plot_backend! function (for when both backends are loaded), and routing wrappers for each conflicting function that forward to the backend-specific 3-argument method.

VortexStepMethodMakieExt.jl — Added init() that sets _PLOT_BACKEND[] = MakieBackend() when the extension loads. Changed all 5 conflicting methods to take ::VortexStepMethod.MakieBackend as a required positional argument, making them distinct from the ControlPlots methods.

VortexStepMethodControlPlotsExt.jl — Same pattern: init() sets _PLOT_BACKEND[] = ControlPlotsBackend(), and all 5 conflicting methods take ::VortexStepMethod.ControlPlotsBackend.

The existing API (plot_geometry(body, title; kwargs...)) continues to work unchanged — the routing wrapper in the main module dispatches to the correct backend automatically. When both backends are loaded, users can switch with set_plot_backend!(MakieBackend()) or set_plot_backend!(ControlPlotsBackend()).