Clamp default frequency grid to Nyquist for discrete diskmargin and mu

[baggepinnen]

Summary

• sim_diskmargin(L, σ, l, u) and structured_singular_value(M0::LTISystem) (no-w form) both used a default frequency grid up to 1e3 rad/s. For discrete-time inputs whose Nyquist π/Ts is below 1e3, the grid extended past Nyquist and freqresp aliased (cis(ω·Ts) is 2π/Ts-periodic), so the worst-case margin / μ from the default grid reflected an aliased frequency rather than the intended one.
• Clamp the upper bound to min(u, π/L.Ts) when the system is discrete. The MIMO branch of diskmargin(L, σ; l, u) benefits automatically since it forwards to sim_diskmargin. Continuous-time behavior is unchanged. Users who deliberately want frequencies past Nyquist can still pass an explicit w vector.

Test plan

• New regression test in test_diskmargin.jl with Ts=1.0 (Nyquist ≈ 3.14 rad/s, well below the default 1e3): asserts the default-grid result agrees with an explicit Nyquist-respecting grid and that the worst-case frequency is at or below Nyquist.
• Existing fast-sampled discrete test (Ts=1e-3, Nyquist ≈ 3141 > 1e3) is unaffected because the clamp resolves to min(1e3, 3141) = 1e3.
• Continuous-time paths unchanged.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.78%. Comparing base (14a7deb) to head (559767e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #148      +/-   ##
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- Coverage   91.84%   91.78%   -0.06%     
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  Files          20       20              
  Lines        3064     3066       +2     
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  Hits         2814     2814              
- Misses        250      252       +2     

Flag	Coverage Δ
unittests	91.78% <100.00%> (-0.06%)	⬇️

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