Raise BVProblemLibrary DiffEqBase compat floor to 6.158 (fix downgrade CI)

[ChrisRackauckas-Claude]

Problem

The downgrade-sublibraries / test (lib/BVProblemLibrary) job is red on master. It fails with:

ERROR: LoadError: UndefVarError: `BVPFunction` not defined
  @ lib/BVProblemLibrary/src/linear.jl:22

Root cause (downgrade)

lib/BVProblemLibrary/Project.toml declared DiffEqBase = "6, 7". The downgrade resolver therefore selected DiffEqBase 6.0.0, an ancient release that:

• predates BVPFunction (re-exported into DiffEqBase from SciMLBase), which src/linear.jl and src/BVProblemLibrary.jl use throughout; and
• does not even precompile on Julia 1.10 (Base.@_pure_meta import failure, combine_axes method-overwriting-during-precompilation error).

From the registry, DiffEqBase only requires julia = "1.10" starting at 6.158, and 6.158's SciMLBase dependency floor (2.60.0) already provides BVPFunction.

Fix 1 — raise DiffEqBase floor

Raise the lower bound: DiffEqBase = "6.158, 7". Raising a floor never loosens anything; it only excludes the broken-on-1.10 ancient versions. The 7.x line is unchanged.

Fix 2 — add Pkg compat (Aqua deps_compat on Julia 1.12)

Touching lib/BVProblemLibrary/Project.toml makes the sublibraries / lib/BVProblemLibrary [QA] job run, which exposed a pre-existing latent Aqua finding on Julia 1.12 (it does not fire on 1.10):

BVProblemLibrary does not declare a compat entry for the following extras: Pkg
Aqua deps_compat: Test Failed ... isempty(result) -> Pkg

Pkg is in [extras]/[targets] (used by test/runtests.jl to activate the qa env) but had no [compat] entry. Added Pkg = "1.10", matching the existing Markdown/Test stdlib pins. Real fix, no test weakening.

Local verification

• Julia 1.10 (downgrade channel): at the new floor pinned to its minimum (DiffEqBase 6.158 + SciMLBase 2.60.0) BVProblemLibrary precompiles and loads, prob_bvp_linear_1 is a BVProblem{..., BVPFunction{...}}, and Pkg.test() passes. DiffEqBase 6.157 is correctly rejected (empty intersection ... 6.158.0-7), confirming the floor is tight.
• Julia 1.12.6 (the failing QA job): DIFFEQPROBLEMLIBRARY_TEST_GROUP=QA Pkg.test() for lib/BVProblemLibrary now reports Aqua | 10 10 (was 9 pass / 1 fail before adding the Pkg compat).

Please ignore until reviewed by @ChrisRackauckas