QA: run_qa v1.6 form + ExplicitImports

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas.

Brings test/qa onto the SciMLTesting 1.6 run_qa form with ExplicitImports enabled. The two hand-rolled files (aqua_tests.jl, jet_tests.jl) collapse into a single qa.jl.

ExplicitImports findings (6 checks, resolved FIX > IGNORE; 0 hard FAIL/BROKEN)

Fixed (4 checks now pass clean):

• no_stale_explicit_imports: dropped stale SMatrix (imported, never used).
• no_implicit_imports: made the 11 implicit LinearAlgebra names explicit — using LinearAlgebra: LinearAlgebra, Adjoint, Cholesky, Diagonal, I, LU, Transpose, UniformScaling, axpby!, axpy!, ldiv!.
• all_qualified_accesses_via_owners: Broadcast.unalias → Base.unalias (unalias is owned by Base).
• all_explicit_imports_via_owners: already passing.

Ignored (other packages'/Base internals, all method-extension imports; go public as base libs declare them):

• all_qualified_accesses_are_public: Base / Base.Broadcast / Base.Iterators internals (@propagate_inbounds, Bottom, BroadcastStyle, Generator, OneTo, ProductIterator, ReshapedArray, axistype, broadcast_unalias, combine_axes, dataids, elsize, maybeview, print_array, print_matrix, to_index, unalias, unsafe_convert), plus non-public names from LinearAlgebra (BlasInt, lutype), Adapt (adapt_storage, adapt_structure), ChainRulesCore (backing), ArrayInterface (indices_do_not_alias, instances_do_not_alias, lu_instance, parent_type), StaticArrayInterface (static_size), Functors (functor). Each ignore is annotated with its source package.
• all_explicit_imports_are_public: StaticArray from StaticArraysCore (not declared public there yet).

No ei_broken needed — every EI check is FIX or IGNORE.

Aqua / JET

• Aqua ambiguities and unbound_args stay disabled via aqua_kwargs (real, long-standing findings in the vcat/hcat/getindex/Axis overloads — genuine design realities, not tracked-broken placeholders; preserves the prior Aqua.test_all(..., ambiguities=false, unbound_args=false) behavior).
• JET runs hard (test_package, clean on Julia 1.10), replacing the prior targeted @report_opt/@report_call set (the whole-package check is broader).

Deps (test/qa/Project.toml)

• Dropped Aqua (provided transitively by SciMLTesting; the ambiguities child-proc requirement doesn't apply since it's disabled) and the transitive ExplicitImports.
• Bumped SciMLTesting compat to "1.6".

Verified locally (Julia 1.10, released SciMLTesting 1.6.0, no dev-from-branch)

• GROUP=QA via run_tests: 16 pass, 0 fail, 0 error, 0 broken.
• ExplicitImports nested testset: 6/6 pass.
• GROUP=Core: all files pass (pre-existing @test_broken in broadcasting_tests unchanged), confirming the explicit LinearAlgebra imports and Base.unalias don't change runtime behavior.

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