QA: tracked-broken Aqua ambiguities + ExplicitImports no_implicit_imports

Tracking issue for the two QA findings that are marked known-broken in test/qa/qa.jl after the move to the SciMLTesting run_qa v1.6 form with ExplicitImports enabled. Both are @test_broken placeholders so the QA lane stays green; they should be fixed and the markers dropped.

1. Aqua ambiguities (aqua_broken = (:ambiguities,))

Aqua.test_all reports 19 method ambiguities, all in this package's own methods:

• ldiv!(::QR{T, <:AlmostBandedMatrix, ...}, ::AbstractVecOrMat{T}) (src ldiv! loop, line ~509) vs ArrayLayouts/LinearAlgebra ldiv!(::Factorization, ::Triangular...) overloads (17 ambiguities).
• __arguments(::AlmostBandedMatrix, ...) overloads (lines ~532/536/537) ambiguous among themselves (2 ambiguities).

These predate this PR; they were silently suppressed by the prior Aqua.test_all(...; ambiguities = false). The v1.6 conversion preserves the suppression as a visible, counted @test_broken rather than a silent disable. Fix: disambiguate the ldiv!/__arguments method tables (e.g. add the specific intersection methods Aqua suggests, or narrow the signatures).

2. ExplicitImports no_implicit_imports (ei_broken = (:no_implicit_imports,))

31 names are used via implicit imports through the package's
using ArrayInterface, ArrayLayouts, BandedMatrices, ConcreteStructs, LazyArrays, LinearAlgebra, MatrixFactorizations, Reexport plus @reexport using BandedMatrices. The full list (from ExplicitImports.print_explicit_imports):

• ArrayLayouts: LayoutMatrix, Lmul, TriangularLayout
• BandedMatrices: BandedMatrix, bandwidth, bandwidths, brand
• ConcreteStructs: @concrete
• LazyArrays: LazyArray, Ldiv, Mul
• LinearAlgebra: LowerTriangular, NoPivot, UnitLowerTriangular, UnitUpperTriangular, UpperTriangular, diagind, lmul!, lu, qr, rank, triu!
• (plus the bare module names ArrayInterface, ArrayLayouts, BandedMatrices, ConcreteStructs, LazyArrays, LinearAlgebra, MatrixFactorizations, Reexport, @reexport)

This is deferred (not done in the conversion PR) because it is a large, mechanical-but-broad using X: a, b, c refactor across 8 dependencies, and the @reexport using BandedMatrices re-export must be preserved. Should be done as its own focused PR.

Opened as part of the SciMLTesting run_qa v1.6 + ExplicitImports rollout.

Start with test/qa/qa.jl and run the QA checks to reproduce the tracked failures. Review the ldiv! loop near src line 509 and the __arguments methods near lines 532, 536, and 537, then audit the listed imports across the package. Done means the Aqua and ExplicitImports markers can be removed while preserving the @reexport using BandedMatrices behavior.