Add ExplicitImports.jl checks to QA via SciMLTesting run_qa

[ChrisRackauckas-Claude]

Summary

Adds ExplicitImports.jl testing to the QA test group through SciMLTesting's run_qa, alongside the existing Aqua and JET checks. All six ExplicitImports checks are enabled:

Standard checks

• no_implicit_imports
• no_stale_explicit_imports
• all_explicit_imports_via_owners
• all_qualified_accesses_via_owners

Public-API checks

• all_qualified_accesses_are_public
• all_explicit_imports_are_public

Result: all six pass with zero changes to package source

ConcreteStructs has no non-stdlib dependencies. Its source uses no using/import of external modules and references Base names only via fully-qualified, public accesses (Base.@__doc__, Base.show, Base.size, etc.). Consequently:

• FIX: none needed — no implicit imports, no stale explicit imports, no non-public qualified accesses.
• DECLARE-PUBLIC: none needed — the only public name is @concrete, already exported.
• IGNORE-LIST: empty — no unavoidable non-public dep names.

Changes

• test/qa/qa.jl: convert the hand-written Aqua.test_all / JET.test_package @testsets to the canonical run_qa(ConcreteStructs; Aqua=Aqua, JET=JET, jet=true, ExplicitImports=ExplicitImports, explicit_imports=true, ...). JET's deprecated target_defined_modules=true is replaced with the supported target_modules=(ConcreteStructs,).
• test/qa/Project.toml: add ExplicitImports (compat 1.4) and bump SciMLTesting compat to 1.4 (the version whose run_qa supports ExplicitImports).

Local verification (Julia 1.12.6 and 1.10.11 / lts)

Full QA group via the CI path (Pkg.test() with GROUP=QA, Julia 1):

Test Summary: | Pass  Total   Time
QA/qa.jl      |   18     18  25.2s
     Testing ConcreteStructs tests passed

The six ExplicitImports checks individually (Julia 1.12.6):

Test Summary:                       | Pass  Total  Time
ExplicitImports (6 checks)          |    6      6  3.1s
  no_implicit_imports               |    1      1  0.5s
  no_stale_explicit_imports         |    1      1  0.8s
  all_explicit_imports_via_owners   |    1      1  0.4s
  all_qualified_accesses_via_owners |    1      1  0.6s
  all_qualified_accesses_are_public |    1      1  0.3s
  all_explicit_imports_are_public   |    1      1  0.4s

Same six pass on Julia 1.10.11 (lts). Runic check passes on test/qa/qa.jl.

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This PR should be ignored until reviewed by @ChrisRackauckas.

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