Bring the QA group onto SciMLTesting 1.6's run_qa with ExplicitImports
enabled, replacing the hand-rolled Aqua.test_all(...; ambiguities=false).

test/qa/qa.jl:
- run_qa(FastAlmostBandedMatrices; explicit_imports = true, ...).
- aqua_broken = (:ambiguities,): preserves the prior `ambiguities = false`
  silent disable as a visible, counted @test_broken placeholder (19 real
  ambiguities in this package's own ldiv!/__arguments). Tracked in SciML#71.
- ei_broken = (:no_implicit_imports,): 31 names used via implicit imports
  through the heavy `using` set + `@reexport using BandedMatrices`; the
  explicit-import refactor is deferred to its own PR. Tracked in SciML#71.
- ei_kwargs ignore-lists for all_explicit_imports_are_public and
  all_qualified_accesses_are_public: upstream non-public names this package
  legitimately extends/uses (ArrayLayouts/BandedMatrices/MatrixFactorizations
  internal QR APIs; Base/LinearAlgebra/LazyArrays/ArrayInterface internals).

src/FastAlmostBandedMatrices.jl (ExplicitImports FIXes -> 0 hard FAIL):
- Remove 4 stale explicit imports (no_stale_explicit_imports): ArrayLayouts
  sub_materialize/zero!/_copyto!, BandedMatrices bandeddata.
- Import QRPackedQLayout/AdjQRPackedQLayout from their owner ArrayLayouts
  instead of re-export MatrixFactorizations (all_explicit_imports_via_owners).

test/qa/Project.toml:
- Drop Aqua: with ambiguities disabled, the Aqua.test_ambiguities child
  process never runs, so Aqua need not be a direct dep (run_qa uses
  SciMLTesting's own Aqua). SafeTestsets kept (run_tests wraps the group body
  in @safetestset).
- SciMLTesting compat -> "1.6".

Verified locally against released SciMLTesting v1.6.0 (Julia 1.10 and 1.11):
QA group 15 Pass / 2 Broken / 0 Fail / 0 Error; Core group unaffected
(28+2 broken core_tests, 11 alloc_tests).

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>