An axis is measured by its length, whatever iterates it

[sshin23]

First fix of the release-candidate iteration (per the PR-per-fix loop).

COPS field testing supplied two iterator types in sequence: catmix's collocation table (fixed by _length(::AbstractArray) in #313) and then steering's zip(2:4, angles) boundary conditions — the same failure shape, a MethodError swallowed into a status-2 layout query that leaves the model unconstructable through the consumer while compiling clean. Enumerating iterator types is the losing game, so this generalizes the rule: a number is the count itself, a symbolic node defers, every other axis answers length.

Verified by sweeping all seventeen COPS models (each built with its own _args defaults): every block of every model resolves to integer dims. Regression test covers the zip axis beside the data axis. Gates: ExaModels 1623 / 0 failed / 2 broken, ExaModelsCompiler 252/252.

Unblocks: steering (the last of Nabla's original six), and COPS's full 35-model verification round.

🤖 Generated with Claude Code

[github-actions]

Your PR requires formatting changes to meet the project's style guidelines.

Please run:

julia --project=@runic -e 'using Pkg; Pkg.add("Runic")'
julia --project=@runic -e "using Runic; exit(Runic.main(ARGS))" -- --fix <files>

(or git runic main if you have the git wrapper installed)

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[github-actions]

Benchmark Results

Relative timing: current / main  (values < 1.0 are improvements)

================================================================================
  backend-instance-param          |      obj     cons     grad      jac     hess
================================================================================
  CUDA-OPF-case1354               |    1.003    1.009    1.035    1.001    1.003
  CUDA-OPF-case14                 |    0.957    0.961    0.956    0.890    0.998
  CUDA-OPF-case30000              |    1.040    1.009    0.901    1.003    1.007
  CUDA-chain-10                   |    1.001    0.995    0.996    1.003    0.997
  CUDA-chain-100                  |    1.011    0.996    0.980    1.027    1.090
  CUDA-chain-1000                 |    0.987    1.002    0.997    0.998    0.999
  CUDA-elec-10                    |    1.022    1.000    0.976    1.001    1.009
  CUDA-elec-100                   |    1.003    1.007    1.136    1.003    1.000
  CUDA-elec-1000                  |    1.022    0.996    0.609    1.037    1.028
  CUDA-rosenrock-1000             |    0.979    0.985    0.964    0.988    0.969
  CUDA-rosenrock-10000            |    1.013    0.948    0.959    0.996    0.965
  CUDA-rosenrock-100000           |    1.002    0.957    0.962    0.988    0.981
--------------------------------------------------------------------------------
  AMDGPU-OPF-case1354             |    1.003    1.008    0.985    0.974    0.998
  AMDGPU-OPF-case14               |    0.997    0.997    0.991    0.992    1.005
  AMDGPU-OPF-case30000            |    0.996    0.992    0.952    0.964    0.979
  AMDGPU-chain-10                 |    0.996    1.001    1.003    0.980    1.020
  AMDGPU-chain-100                |    0.999    0.992    0.998    0.997    1.039
  AMDGPU-chain-1000               |    1.001    1.028    0.994    0.958    1.015
  AMDGPU-elec-10                  |    1.002    1.008    1.003    1.048    1.002
  AMDGPU-elec-100                 |    0.997    0.993    0.979    0.982    0.989
  AMDGPU-elec-1000                |    0.996    1.007    1.039    0.988    0.966
  AMDGPU-rosenrock-1000           |    0.990    1.032    0.988    0.964    1.042
  AMDGPU-rosenrock-10000          |    0.978    1.034    1.005    0.973    1.048
  AMDGPU-rosenrock-100000         |    0.924    0.964    0.955    0.964    1.305
--------------------------------------------------------------------------------
  oneAPI-OPF-case1354             |    0.863   17.400    4.344    1.251    0.975
  oneAPI-OPF-case14               |    1.089    0.753    0.847    0.523    1.429
  oneAPI-OPF-case30000            |    0.743    0.662    2.718    1.410    4.356
  oneAPI-chain-10                 |    0.575    3.566    1.659    1.372    1.126
  oneAPI-chain-100                |    0.822    1.655    1.168    5.967    1.265
  oneAPI-chain-1000               |    0.695    1.448    1.341    0.957    1.435
  oneAPI-elec-10                  |    1.087    2.278    0.850    1.249    1.987
  oneAPI-elec-100                 |    4.141    2.361    1.316   25.341    1.012
  oneAPI-elec-1000                |    1.017    1.395    0.969    1.961    2.562
  oneAPI-rosenrock-1000           |    0.710    1.251    1.308    0.139    1.210
  oneAPI-rosenrock-10000          |    1.198    1.315    1.448    0.091    0.759
  oneAPI-rosenrock-100000         |    0.368    1.200    0.785    1.118    0.980
--------------------------------------------------------------------------------
  nothing-OPF-case1354            |    1.094    1.000    1.010    0.975    1.059
  nothing-OPF-case14              |    0.979    0.984    1.009    1.012    1.374
  nothing-OPF-case30000           |    0.995    0.984    0.980    0.988    1.042
  nothing-chain-10                |    0.967    0.907    0.676    0.874    0.994
  nothing-chain-100               |    0.971    0.988    0.750    0.944    0.717
  nothing-chain-1000              |    1.000    1.053    0.818    0.998    0.636
  nothing-elec-10                 |    1.031    1.163    1.035    1.037    1.007
  nothing-elec-100                |    0.999    1.157    1.001    0.986    1.002
  nothing-elec-1000               |    1.000    1.176    0.984    0.993    1.022
  nothing-rosenrock-1000          |    0.996    0.999    1.003    0.963    1.211
  nothing-rosenrock-10000         |    1.000    0.998    1.000    0.987    0.303
  nothing-rosenrock-100000        |    1.056    1.041    1.057    0.991    0.878
================================================================================