Update to ModelingToolkit v11 requires code migration

[ChrisRackauckas-Claude]

Summary

PR #100 attempts to add ModelingToolkit v11 compat for NeuralLyapunovProblemLibrary, but this requires several breaking changes to be addressed.

Problem

ModelingToolkit v11 introduces breaking API changes:

1. Incompatible Symbolics versions:

   • MTK 9.x requires Symbolics 6.x
   • MTK 11.x requires Symbolics 7.x
   • These cannot coexist - we must choose one

2. structural_simplify API changes:

   • Old (MTK 9): structural_simplify(sys, (inputs, outputs); split=..., simplify=...) returns a tuple
   • New (MTK 11): mtkcompile(sys; inputs=..., outputs=...) returns a single System

3. Other API changes per the MTK v10 migration guide:

   • ODESystem → unified System type
   • Callback semantics changed
   • getproperty/setproperty! for system fields removed

Files Affected

The following test files use the old structural_simplify API with tuple unpacking:

• test/pendulum_test.jl (lines 35-41, 59)
• test/double_pendulum_test.jl (lines 90, 115, 181, 214)
• test/planar_quadrotor_test.jl (lines 26, 57, 123, 159)
• test/quadrotor_test.jl (lines 35, 65, 156, 194)

Recommended Next Steps

1. Drop MTK 9 support - Update compat to ModelingToolkit = "11" only
2. Update Symbolics compat - Change to Symbolics = "7"
3. Migrate all test files to use the new mtkcompile API
4. Update main NeuralLyapunov package if it also needs MTK 11 support

Alternative

If MTK 9 support is still needed, we may need to wait until:

• Upstream provides a compatibility layer
• Or create conditional code paths for both versions (complex)

cc @ChrisRackauckas

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