Audit public reexports and inherited module docs

[ChrisRackauckas-Claude]

Ignore until reviewed by @ChrisRackauckas.

This PR handles public reexports in two separate ways:

• Re-exported dependency modules inherit their defining package's module documentation and are not forced into a local @docs block. Re-exported functions and types still require a local rendered API entry.
• run_qa gains an opt-in check_reexports = true audit. It remains off by default to avoid a breaking fleet-wide change. Intentional facade API is listed with reexports_allow.
• The exported public_reexports helper detects imported functions, types, modules, constants, macros, and operators, plus aliases whose values expose module ownership.
• Package-owned APIs defined in nested submodules are not classified as dependency reexports.
• The version is bumped to 2.4.0 because this adds public API.

Julia cannot recover source-binding provenance after locally assigning scalar/enum values, callable objects, or constructed Union types; direct imports are fully covered.

Local verification:

• Julia 1.10: Pkg.test() passed, 318/318.
• Julia 1.12: Pkg.test() passed, 333/333.
• Runic and git diff --check passed.

[ChrisRackauckas-Claude]

I validated this PR's current head (a5ccb593f0f2551b9a325da8960701b754dfd004) against the exact OptimizationOptimisers_QA harness on Optimization.jl master.

It improves the result substantially, reducing the rendered-public-API failure from 300 names to nine, but the harness still fails:

DEFAULT_CALLBACK
DEFAULT_DATA
DebugLevel
ErrorLevel
InfoLevel
MaxSense
MinSense
Silent
WarnLevel

These are inherited values/constants or enum values. parentmodule(getfield(pkg, name)) !== pkg identifies inherited functions and types, but the value itself does not retain the binding's source module in these cases.

Reproduction setup:

• Optimization.jl 34a8fd5cb3484c1c7ce471f7964d0bf81bc9796a
• Julia 1.12.6
• this PR checked out as the local SciMLTesting source
• GROUP=OptimizationOptimisers_QA JULIA_PKG_PRECOMPILE_AUTO=0 julia +1.12 --startup-file=no --project=. -e 'using Pkg; Pkg.test()'

The analogous OptimizationPolyalgorithms_QA lane also fails under released SciMLTesting 2.3.0, with 264 inherited names. I recorded the full boundary and CI evidence in SciML/Optimization.jl#1278.