docs: update MTK example for ModelingToolkit v11

[baggepinnen]

The ModelingToolkit example (docs/src/examples/modelingtoolkit.md) claimed to be tested with MTK v10.0.1. On the current release (v11.36.0) it no longer runs, because the @mtkmodel DSL was removed in favor of the functional component form.

Changes to make the example run on MTK v11.36.0

• @mtkmodel QuadtankModel begin ... end → @component function QuadtankModel(; name) ... System(eqs, t; name) end (the @equations block becomes a plain eqs = [...] vector, and the intermediate begin ... end block of local assignments becomes ordinary code).
• Array-variable defaults must now match the variable size: h(t)[1:4] = 0 → h(t)[1:4] = zeros(4), u(t)[1:2] = 0 → zeros(2).
• ModelingToolkit.defaults(io_sys)[pi] → ModelingToolkit.getdefault(pi) (defaults is no longer exported/available).

Verification

Ran the full example end-to-end on MTK v11.36.0. The estimated parameters still recover the true values p_true = [1.6, 1.6, 4.9, 0.2]:

p: [1.611987373192685, 1.599185391084847, 4.877360360086102, 0.2044771473506988]

The printed output block, the @btime timing, and the tested-version note in the warning admonition are updated to match.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.49%. Comparing base (f1af47c) to head (49e8a04).

Additional details and impacted files

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##           master     #200   +/-   ##
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  Coverage   89.49%   89.49%           
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  Files          15       15           
  Lines        2722     2722           
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  Hits         2436     2436           
  Misses        286      286           

Flag	Coverage Δ
unittests	89.49% <ø> (ø)

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