Fix ifelse support and update ModelingToolkit compat

[ChrisRackauckas-Claude]

Summary

This PR fixes issue #36 by addressing two related problems:

1. Added ifelse helper function for MATLAB

   • MATLAB doesn't have a built-in ifelse function, but Symbolics.jl generates code using ifelse(cond, a, b) for the MATLABTarget()
   • We now define ifelse = @(cond, a, b) cond .* a + ~cond .* b; in MATLAB before evaluating the generated code
   • This allows symbolic ifelse expressions (like ifelse(a > 0, -a, a)) to work correctly

2. Updated ModelingToolkit compat to support versions 8, 9, 10, 11

   • The original error TypeError: non-boolean (Num) used in boolean context was fixed in newer ModelingToolkit/Symbolics versions
   • Added compatibility shim for the states -> unknowns API rename (v9+)

Background

The original issue used this MWE:

using MATLABDiffEq
using ModelingToolkit
using OrdinaryDiffEq
@variables t
@variables a(t) = 1.0
D = Differential(t)
@named sys = ODESystem([D(a) ~ ifelse(a>0.0,-a,a)])
prob = ODEProblem(sys, [], (0.0,1.0))
solve(prob, MATLABDiffEq.ode15s())

Which failed with TypeError: non-boolean (Num) used in boolean context.

Test plan

• CI tests should pass (MATLAB not available in CI, but syntax was verified)
• Manual testing with MATLAB required to fully verify the fix

cc @ChrisRackauckas

🤖 Generated with Claude Code

[ChrisRackauckas-Claude]

CI Failure Analysis

The CI failure is NOT related to my changes. The Downgrade workflow is failing due to a dependency resolution issue:

ERROR: Unsatisfiable requirements detected for package RecursiveArrayTools [731186ca]:
 RecursiveArrayTools [731186ca] log:
 ├─possible versions are: 0.16.0-3.42.1 or uninstalled
 └─restricted to versions 2.39.0 by an explicit requirement — no versions left

Looking at recent CI runs on master, the same failure occurs:

• Recent master runs (20565352669, 20565346985) also show failure status

This appears to be a pre-existing CI configuration issue with the julia-downgrade-compat action creating unsatisfiable constraints.

The actual code changes are:

1. Added ifelse helper function for MATLAB (MATLAB doesn't have built-in ifelse)
2. Added compatibility shim for states -> unknowns API rename in ModelingToolkit v9+
3. Updated ModelingToolkit compat to support versions 8-11

These changes are straightforward and syntactically correct (verified via Meta.parse).

cc @ChrisRackauckas