qa: import names through owning modules

[ChrisRackauckas-Claude]

Ignore this PR until it has been reviewed by @ChrisRackauckas.

What changed

Import names through the modules that own them according to Base.which and the strict ExplicitImports owner checks:

• OrdinaryDiffEq extension modules import t_nounits and D_nounits from ModelingToolkitBase.
• ModelingToolkit imports and qualifies unwrap through SymbolicUtils.
• scalarize remains imported from Symbolics, where it is public.

This is a focused QA correctness fix. It does not change algorithms, precompile workloads, public API, or documentation.

Failure before

On this branch before the scalarize correction:

GROUP=QA julia --project -e 'using Pkg; Pkg.test()'
Some tests did not pass: 49 passed, 0 failed, 3 errored, 0 broken.
all_explicit_imports_via_owners: passed
all_qualified_accesses_via_owners: passed
all_qualified_accesses_are_public: passed
all_explicit_imports_are_public: NonPublicExplicitImportsException
- scalarize is not public in SymbolicUtils

Verification after

With the correction applied:

GROUP=QA julia --project -e 'using Pkg; Pkg.test()'
Some tests did not pass: 50 passed, 0 failed, 2 errored, 0 broken.
all_explicit_imports_via_owners: passed
all_qualified_accesses_via_owners: passed
all_qualified_accesses_are_public: passed
all_explicit_imports_are_public: passed

The two remaining errors are the pre-existing analyzer limitation caused by the dynamic include(pkgdir(ModelingToolkitBase, "src", "precompile.jl")) in src/precompile.jl: no_implicit_imports and no_stale_explicit_imports report UnanalyzableModuleException. This PR does not suppress or allow-list those failures.

Additional checks:

• Runic check passed for all changed Julia files.
• typos passed on changed files.
• git diff --check passed.
• No docs build was run because this PR does not touch documentation or docstrings.

This is independent of the cross-reference work in #4980.