Document public API docs coverage

[ChrisRackauckas-Claude]

This PR should be ignored until reviewed by @ChrisRackauckas.

Summary

• Adds source docstrings for package-owned public API bindings that lacked documentation: AbstractAxis, FlatAxis, Shaped1DAxis, deprecated fastindices, and @static_unpack.
• Enables SciMLTesting public API docs QA with run_qa(...; api_docs_kwargs = (; rendered = true)).
• Pins the QA environment to SciMLTesting 2.1 from SciML/SciMLTesting.jl commit a5cecca928f2c684c23505c3cd83921d71753e2e, since the helper is not registered yet.
• Runs the QA group in its pinned QA project so the root test environment does not pre-load registered SciMLTesting 1.x before the API-docs check.

Validation

• git diff --check
• GROUP=QA timeout 3600 /home/crackauc/.juliaup/bin/julia +1.10 --project=. -e 'using Pkg; Pkg.test()' (Quality Assurance: 16 pass)
• GROUP=QA timeout 3600 /home/crackauc/.juliaup/bin/julia +1.11 --project=. -e 'using Pkg; Pkg.test()' (Quality Assurance: 18 pass)
• timeout 3600 /home/crackauc/.juliaup/bin/julia +1.11 --project=. -e 'using Pkg; Pkg.test()'
• timeout 3600 /home/crackauc/.juliaup/bin/julia +1.11 --project=docs -e 'using Pkg; Pkg.instantiate(); push!(LOAD_PATH, pwd()); include("docs/make.jl")' (passed; local Documenter deploy warning only)
• timeout 3600 /home/crackauc/.juliaup/bin/julia +1.11 --project=<temporary local Runic env> -e 'using Pkg; Pkg.add(PackageSpec(name="Runic")); using Runic; exit(Runic.main(["--check", "--diff", "src", "ext", "test", "docs", "examples"]))'

CI Note

• Current PR CI passes for docs, format, downgrade, QA, core, GPU, Autodiff, and Reactant.
• Downstream fails on Julia 1 and LTS with the same LinearAlgebra.LU type mismatch in test/Downstream/diffeq_tests.jl:66; the LTS form reproduces on current main (actions/runs/29074867034, job 86308981605), so this is independent of the docs/QA changes here.

[ChrisRackauckas-Claude]

Pushed the SciMLTesting v2.1 cleanup commit (7621ba0).

Local validation after the cleanup:

• Pkg.test() passed
• GROUP=QA Pkg.test() passed
• GROUP=Autodiff Pkg.test() passed
• GROUP=GPU Pkg.test() passed
• GROUP=Reactant Pkg.test() passed
• git diff --check passed
• julia -m Runic --check test/runtests.jl passed
• TOML parsing for edited test environments passed
• no test Project.toml contains package path = ... sources

GROUP=Downstream Pkg.test() fails, but this reproduces on clean main at cc4cacf with SciMLTesting v1.8.0 before this branch. The failure is test/Downstream/diffeq_tests.jl:66 / Issue 36: TypeError: in setfield!, expected LinearAlgebra.LU{Float64, ComponentMatrix...}, got LinearAlgebra.LU{Float64, Matrix{Float64}, Vector{Int64}}. A separate subagent is continuing that base-failure investigation.