Document public API coverage

[ChrisRackauckas-Claude]

Draft PR. Ignore until reviewed by @ChrisRackauckas.

Summary

• Documented the public SparseMatrixIdentification API at definition sites.
• Enabled Documenter exported-doc coverage checking with checkdocs = :exports.
• Switched QA public API docs coverage to SciMLTesting run_qa(...; api_docs_kwargs = (; rendered = true)).
• Pinned QA SciMLTesting to https://github.com/SciML/SciMLTesting.jl at a5cecca928f2c684c23505c3cd83921d71753e2e with compat SciMLTesting = "2.1".
• Isolated the QA subprocess so root tests that load registered SciMLTesting 1.8 can still run the pinned QA environment with run_api_docs.

Public API Audit

• export: getstructure, sparsestructure.
• Julia public / @public / SciMLPublic: none found.
• Rendered docs include both exports plus the extension hooks already listed in docs/src/api.md.

Validation

• timeout 3600 julia +1.10 -e 'using Runic; exit(Runic.main(ARGS))' -- --check .
• timeout 3600 julia +1.10 --project=. -e 'using Pkg; Pkg.test()'
• GROUP=QA timeout 3600 julia +1 --project=. -e 'using Pkg; Pkg.test(; coverage=false, julia_args=["--check-bounds=yes", "--compiled-modules=yes", "--depwarn=yes"], force_latest_compatible_version=false, allow_reresolve=true)'
• timeout 3600 julia +1 --project=test/qa -e 'using Pkg; Pkg.instantiate(); Pkg.develop(PackageSpec(path=pwd())); import SciMLTesting; @assert isdefined(SciMLTesting, :run_api_docs); include("test/qa/qa.jl")'
• timeout 3600 julia +1.10 --project=docs -e 'using Pkg; Pkg.develop(PackageSpec(path=pwd())); Pkg.instantiate(); include("docs/make.jl")'

Note: direct test/qa instantiate on Julia 1.10 does not honor [sources] for unregistered SciMLTesting 2.1; the package lts tests pass on Julia 1.10, and the QA lane validates on Julia 1 / 1.12 where the pinned [sources] entry resolves.