Fix isstable for discrete Dual state-space to check eigenvalues

[baggepinnen]

Summary

The isstable specialization for StateSpace{<:Discrete, <:ForwardDiff.Dual} checked |A[i,j]| < 1 element-wise rather than |λ(A)| < 1. This is unrelated to discrete-time stability.

Example counter-example before the fix:

A = 0.9 * ones(2,2)   # entries all < 1
# eigenvalues: 0.0, 1.8 → not stable, but old code returned true

The continuous-time Dual specialization immediately above already uses eigvals correctly; this aligns the discrete one with it.

Test plan

• Construct a discrete Dual state-space with entries < 1 but spectral radius > 1, confirm isstable returns false.
• Existing tests still pass.

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.61%. Comparing base (0aa6e4c) to head (68a593a).

Files with missing lines	Patch %	Lines
lib/ControlSystemsBase/src/types/StateSpace.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1057   +/-   ##
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  Coverage   91.61%   91.61%           
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  Files          42       42           
  Lines        5653     5653           
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  Hits         5179     5179           
  Misses        474      474           

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