Adopt SciMLTesting 2.4 strict QA

[ChrisRackauckas-Claude]

Summary

• require SciMLTesting 2.4 in root and QA environments
• remove repository-local [sources] paths and QA suppressions
• enable doctests and exported API documentation checks
• move owned API documentation to definition sites

Verification

• git diff --check passed
• Runic formatted every changed Julia file
• strict QA currently exposes non-public imports, blanket reexports, and a pre-existing Aqua piracy report; no ignores, skips, or weakened settings were added
• package tests and strict docs are still running in local isolated depots

Required follow-up

The strict failures need public-API owner changes (especially Symbolics/SymbolicUtils and the ModelingToolkitBase/StateSelection boundaries) plus an explicit, SemVer-major replacement for blanket reexports. They are intentionally not waived here.

Ignore until reviewed by @ChrisRackauckas.

[ChrisRackauckas-Claude]

Strict-QA audit note (2026-07-30):

• The remaining 481 root public reexports originate in the documented @reexport using Symbolics, UnPack, ModelingToolkitBase, and StructuralTransformations façades. Removing the Symbolics façade is a documented-interface decision (using ModelingToolkit is documented to reexport Symbolics), not an undocumented-reexport cleanup. This PR therefore does not add reexports_allow or weaken the check.
• ExplicitImports reports names owned but not public in Symbolics/SymbolicUtils, SciMLBase, ModelingToolkitTearing, OffsetArrays, and Base. These need owner-side public contracts or local replacements; they are not suppressed here. The ModelingToolkitBase-to-root imports additionally need an explicit developer-API boundary.
• Aqua reports extensions owned by SciMLBase/ModelingToolkitBase/StateSelection. Resolving those requires owner-defined extension points rather than an Aqua exception.
• I corrected remaining package-owned Shift and Sample doctest expectations and am validating the full docs build with doctests enabled.

[ChrisRackauckas-Claude]

Follow-up validation:

• JULIA_DEPOT_PATH=.docs-remediation-depot julia +1.12 --project=docs docs/make.jl executed doctests, then failed strict Documenter checks. The corrected Shift and Sample doctests passed; the build proceeded through example execution and cross-reference checks.
• Remaining strict-docs work is substantial and pre-existing: stale/unqualified @docs blocks (including external BipartiteGraphs/SciMLBase/Symbolics references), 43 local docstrings missing canonical manual entries, unresolved cross-references, and dynamic-optimization examples with @variables ambiguity after JuMP/InfiniteOpt loading. A file://.../atomic_array_dict.jl source link is also being linkchecked. None has been downgraded to warnonly or disabled.
• Focused verified commit pushed: 90c168ed09 (Fix discrete operator doctest outputs). Local Runic and git diff --check pass. CI has started; PR remains draft and currently has an unstable merge state while checks run.