QA: run_qa v1.6 form + ExplicitImports

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas.

Converts test/qa/qa.jl to the SciMLTesting 1.6 run_qa form with ExplicitImports enabled:

using SciMLTesting, SparseBandedMatrices, JET, Test

run_qa(SparseBandedMatrices; explicit_imports = true)

This replaces the hand-rolled Aqua + bespoke JET (report_call/report_opt) qa.jl. Aqua now runs test_all with full defaults (the previous ambiguities = (recursive = false,) tweak was a no-op here — test_all passes with defaults), JET runs the canonical test_package (mode = :typo), and the six ExplicitImports checks are enabled.

ExplicitImports findings (all resolved by FIX; nothing ignored or marked broken)

check_no_implicit_imports flagged LinearAlgebra (LinearAlgebra, I, mul!) and PrecompileTools (PrecompileTools, @setup_workload, @compile_workload). Made the imports explicit in src/:

import LinearAlgebra: mul!
using .Threads: @threads
using PrecompileTools: @setup_workload, @compile_workload

• mul! is now extended via the bare imported name (was LinearAlgebra.mul!).
• The I flag was a false positive — it is the varargs parameter I::Int... in getindex/setindex!, not LinearAlgebra.I. It disappears once the blanket using LinearAlgebra is gone.
• All 6 EI checks pass; no ei_kwargs ignores and no ei_broken entries are needed.

No aqua_broken/jet_broken/ei_broken markers (none were present before, and nothing newly fails).

Deps

• test/qa/Project.toml: SciMLTesting compat floor 1 → 1.6; dropped the now-unused LinearAlgebra dep (qa.jl no longer uses it). ExplicitImports stays transitive via SciMLTesting (not added). Aqua / JET / SafeTestsets kept (Aqua's ambiguities child-process + JET + the @safetestset harness need them resolvable in the env).
• Project.toml: bump PrecompileTools compat floor 1 → 1.1. Because the package now explicit-imports only PrecompileTools' macros, the macro expansion references the bare PrecompileTools module name, which is undefined under 1.0 (UndefVarError on load) — verified by pinning 1.0.0. 1.1 is the first self-contained-macro release; verified the package loads at the 1.1 floor.

Verification (local, Julia 1.10, released SciMLTesting 1.6.0)

• QA group: 18/18 pass — Aqua 11, JET 1, ExplicitImports 6. 0 FAIL/ERROR/BROKEN.
• Core group: 41/41 pass (constructors 4, division 3, interface 31, multiplication 3) — confirms the mul! extension change is behavior-preserving.
• Downgrade floor check: package loads cleanly with PrecompileTools pinned to 1.1.0.

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