QA: run_qa v1.6 form + ExplicitImports

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas.

Converts test/qa/qa.jl from the hand-rolled Aqua + ExplicitImports body to the SciMLTesting 1.6 run_qa form and enables the full ExplicitImports suite (explicit_imports = true).

qa.jl

• Single run_qa(Static; ...) call. Aqua.test_all replaces the eight per-sub-check calls (find_persistent_tasks_deps, test_ambiguities, test_deps_compat, test_piracies, test_project_extras, test_stale_deps, test_unbound_args, test_undefined_exports — all covered by test_all).
• Aqua.test_ambiguities(..., recursive = false) preserved via aqua_kwargs = (; ambiguities = (; recursive = false)).
• Aqua.test_piracies(..., broken = true) mapped to aqua_broken = (:piracies,) (disables the sub-check in test_all + emits a tracked @test_broken placeholder).

ExplicitImports findings (now all six checks run)

Previously only no_implicit_imports + no_stale_explicit_imports ran. Enabling all six:

• Pass bare (4): no_implicit_imports, no_stale_explicit_imports, all_explicit_imports_via_owners, all_qualified_accesses_via_owners.
• Ignored (2), other-package not-yet-public names:
  • all_qualified_accesses_are_public: 22 Base / Base.Broadcast / Base.IteratorsMD internals (@propagate_inbounds, tail, OneTo, axistype, to_index, ...). Ignore until Base declares them public.
  • all_explicit_imports_are_public: IfElse.ifelse (exported-but-not-public by IfElse.jl, whose sole purpose is that name).
• Broken (0), fixed (0). No EI check is left failing or @test_broken.

Aqua broken

• piracies: Base.promote_shape overload on Tuple{Vararg{Union{Int,StaticInt}}} subsumes plain Tuple{Vararg{Int}} — genuine Base piracy. Tracked in QA: Aqua piracies finding — Base.promote_shape overload on Tuple{Vararg{Union{Int,StaticInt}}} #181. (The earlier piracy issue piracy in Base.all and Base.any #162, Base.all/Base.any, was a different, now-resolved method; QA: Aqua piracies finding — Base.promote_shape overload on Tuple{Vararg{Union{Int,StaticInt}}} #181 is the remaining one.)

Deps (test/qa/Project.toml)

• Drop ExplicitImports and SafeTestsets (both transitive via SciMLTesting).
• Keep Aqua direct (the ambiguities child-process needs it as a direct dep).
• SciMLTesting compat -> "1.6". No JET (it was not run before).

Verification

Run locally against released SciMLTesting 1.6.0 (Pkg resolves 1.6.0; develop-by-path so master's #180 axistype fix is in scope), Julia 1.10 and 1.12:

Test Summary:     | Pass  Broken  Total   Time
Quality Assurance |   16       1     17

ExplicitImports nested set: 6/6 pass. 0 Fail / 0 Error; the single Broken is the tracked aqua: piracies placeholder.

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