Add polar-axis option to Makie nyquistplot

[baggepinnen]

Adds a polar keyword option to CSMakie.nyquistplot that draws the Nyquist curve in a Makie.PolarAxis instead of a Cartesian Axis. This provides native polar "graph paper": circles of constant magnitude and rays of constant phase, drawn and re-ticked automatically by Makie when the axis limits change, with correct aspect ratio by construction. Requested in discussion JuliaControl/ControlSystems.jl#1064.

A rlimits keyword (forwarded to PolarAxis) allows limiting the radial axis, which is useful for systems with poles on or near the imaginary axis where the Nyquist curve reaches very large magnitudes.

All existing options (Ms_circles, Mt_circles, unit_circle, critical_point, MIMO grid layout, adaptive downsampling) work in polar mode as well; Cartesian coordinates are converted point-wise to (angle, radius).

Example

using ControlSystemsBase, GLMakie  # or CairoMakie

P = tf(4, [1, 2, 1, 0])  # 4 / (s(s+1)²)
CSMakie.nyquistplot(P; polar=true, rlimits=(0, 3), unit_circle=true, Ms_circles=[1.7])

🤖 Generated with Claude Code

https://claude.ai/code/session_01KU255FUfQGyZa2cziDYf7C

[JuliaControlBot]

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After pulling this PR, please update the reference images by creating a PR to ControlExamplePlots.jl here.

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[codecov]

Codecov Report

❌ Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 91.68%. Comparing base (b2b7dc4) to head (169386e).

Files with missing lines	Patch %	Lines
...ntrolSystemsBase/ext/ControlSystemsBaseMakieExt.jl	93.75%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1065      +/-   ##
==========================================
+ Coverage   91.64%   91.68%   +0.04%     
==========================================
  Files          42       42              
  Lines        5660     5665       +5     
==========================================
+ Hits         5187     5194       +7     
+ Misses        473      471       -2     

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