Use oneunit in remaining tf/ss/zpk type-promotion sites

[baggepinnen]

Summary

• Completes the fix started in b394b96. Measurements.jl deliberately defines Base.one(::Type{Measurement{T}}) = one(T), so one(Measurement{Float64}) === 1.0::Float64. Five conversion sites still inferred the result element type via typeof(one(T0)/one(T0)) / typeof(zero(T0)/one(T0)), which collapsed Measurement{Float64} to Float64 and triggered the MethodError: no method matching Float64(::Measurement{Float64}) reported in MatrixPencils - a new feature #1027 along the tf → zpk, ss → tf, ss → zpk, minreal, and charpoly paths.
• Switched the remaining sites to typeof(oneunit(T)/oneunit(T)), matching the existing fix in convert(::Type{StateSpace}, G::TransferFunction). Behavior for Int/Float64/BigFloat/etc. is unchanged (oneunit/oneunit reduces to one/one when one(T) isa T).
• Sites touched: types/conversion.jl (×3: ss→TF/SisoRational, ss→TF/SisoZpk, charpoly), types/zpk.jl (tf→zpk), types/SisoTfTypes/SisoRational.jl (minreal), matrix_comps.jl (_infnorm_two_steps_dt, for consistency).

Closes #1027 to the extent it's a ControlSystems issue. (zpk and tf(::ss{Measurement}) may still fail downstream inside Polynomials.roots / LinearAlgebra.hessenberg, which need BLAS-able elements — separate upstream limitations.)

Test plan

• lib/ControlSystemsBase test suite: 21146 pass, 6 broken (unchanged from master).
• RobustAndOptimalControl.jl test suite (extra promotion coverage): 1357 pass, 6 broken (unchanged).
• Issue MatrixPencils - a new feature #1027 reproducer: ss(P) now returns StateSpace{Continuous, Measurement{Float64}} with elements preserved.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.61%. Comparing base (75801d6) to head (b0b103b).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1055      +/-   ##
==========================================
+ Coverage   91.57%   91.61%   +0.04%     
==========================================
  Files          42       42              
  Lines        5623     5653      +30     
==========================================
+ Hits         5149     5179      +30     
  Misses        474      474              

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