Fix MIMO Ai-disturbance, resonant disturbance shape and docs

[baggepinnen]

Summary

Review-driven cleanup of src/model_augmentation.jl.

• add_low_frequency_disturbance(sys, Ai::Integer): Ad is now 1×1. The previous Ad = -ϵ*I(nu) only worked for SISO plants; for nu > 1 the augmented A = [A Cd; 0 Ad] had mismatched column counts.
• add_resonant_disturbance(sys, ω, ζ, Bd): validate Bd shape with informative errors and accept a 2-column Bd so both resonant states can inject into the plant. The non-measurement branch previously assumed Bd had exactly one column and silently constructed an ill-shaped Cd otherwise.
• add_resonant_disturbance docs: corrected the polynomial / parameter meaning. The implemented Ad = [-ζ -ω; ω -ζ] has eigenvalues -ζ ± iω and characteristic polynomial s² + 2ζs + (ζ² + ω²), not the previously documented s² + 2ζωs + ω². ζ is now described as the decay rate and ω as the damped frequency.
• Element types: Ad/Cd in add_resonant_disturbance now use eltype(sys.A) for consistency with the other augmentation routines.
• add_input_integrator: renamed local Ad/Bd/Cd to A_int/B_int_row/C_int_row to disambiguate from the disturbance-augmentation naming used elsewhere in the file.

Test plan

• julia --project -e 'using LinearAlgebra, RobustAndOptimalControl, Test; include("test/test_augmentation.jl")' — passes, including the new MIMO Ai-integer regression, the new two-column Bd test, and the new Bd-shape validation tests.
• julia --project -e 'using Plots, LinearAlgebra, RobustAndOptimalControl, Test; include("test/test_lqg.jl")' — passes (add_input_differentiator / add_output_integrator callers).
• Full Pkg.test() in CI.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.81%. Comparing base (e199e36) to head (72a5e5a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #141      +/-   ##
==========================================
+ Coverage   91.45%   91.81%   +0.35%     
==========================================
  Files          20       20              
  Lines        3056     3065       +9     
==========================================
+ Hits         2795     2814      +19     
+ Misses        261      251      -10     

Flag	Coverage Δ
unittests	91.81% <100.00%> (+0.35%)	⬆️

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