QA: run_qa v1.6 form + ExplicitImports

[ChrisRackauckas-Claude]

Ignore until reviewed by @ChrisRackauckas.

Brings LightweightStats.jl onto the SciMLTesting run_tests folder model and moves QA onto the run_qa v1.6 form with ExplicitImports enabled.

Changes

• test/runtests.jl -> using SciMLTesting; run_tests() (folder-discovery mode).
• test/test_groups.toml declares Core (lts/1/pre, 3 OSes) + QA (lts/1, ubuntu).
• Core group: top-level test/*.jl are auto-discovered. The inline mean/median/var/... testsets that lived in runtests.jl move to test/core_tests.jl; regression_tests.jl and interface_tests.jl stay as Core files.
• QA group: new test/qa/qa.jl = run_qa(LightweightStats; explicit_imports = true) with JET loaded. Replaces the hand-rolled test/jet_tests.jl + test/explicit_imports_tests.jl.
• test/qa/Project.toml: SciMLTesting "1.6", JET, Test, and Aqua kept as a direct dep (Aqua's ambiguities child-process needs it). ExplicitImports dropped (transitive via SciMLTesting).
• Root [targets].test gains SciMLTesting + SafeTestsets (the @safetestset wrapper the folder model uses needs SafeTestsets resolvable by name) and drops ExplicitImports + JET (now in the QA sub-env). PrecompileTools floor stays 1.2 (the using PrecompileTools: @compile_workload macro-only import).
• CI Tests.yml switches to grouped-tests.yml@v1 so QA runs as its own lane.

ExplicitImports findings

Zero findings — no ignore-lists, no ei_broken, no @test_broken. All 6 checks pass clean (no_implicit_imports, no_stale_explicit_imports, all_explicit_imports_via_owners, all_qualified_accesses_via_owners, all_qualified_accesses_are_public, all_explicit_imports_are_public). The package has zero non-stdlib runtime deps beyond PrecompileTools and only one self-qualified LightweightStats.mean access (benign; not a finding). JET report_package is clean (0 reports), so no jet_broken either.

Verification (released SciMLTesting 1.6.0, no dev-from-branch)

• QA group: 18/18 pass, 0 fail / 0 error / 0 broken on Julia 1.10 (lts) and 1.11 (1).
• Core group on 1.11: core_tests 69/69, interface_tests 33/33, regression_tests 322/322.

🤖 Generated with Claude Code