QA: run_qa v1.6 form + ExplicitImports

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas.

Brings CurveFit.jl's QA onto the SciMLTesting run_qa v1.6 form with ExplicitImports enabled, consolidating the previously scattered hand-rolled QA into a single test/qa/ group wired via the run_tests() folder model.

What changed

• test/qa/qa.jl (new) — run_qa(CurveFit; explicit_imports = true, ei_kwargs = ...). Runs Aqua (test_all, all sub-checks) plus the six ExplicitImports checks. No JET (matches the prior QA, which ran Aqua + EI only). No aqua_broken/jet_broken/ei_broken: every check is a genuine PASS.
• test/qa/Project.toml (new, replaces test/nopre/Project.toml) — Aqua + SafeTestsets + SciMLTesting + Test + CurveFit (path source). ExplicitImports is transitive via SciMLTesting (dropped as a direct test dep). SciMLTesting compat "1.6".
• Removed test/nopre/ (hand-rolled Aqua.test_all group) and test/explicit_imports.jl (raw EI checks that ran inside Core) — both subsumed by run_qa.
• test/test_groups.toml — replace the nopre group with QA (in_all = false, versions 1/lts), matching the QA folder. Same two (group × version) CI cells as before.
• Root Project.toml — drop ExplicitImports from [extras]/[targets].test (only the removed Core file used it), bump SciMLTesting compat to "1.6", and raise the PrecompileTools floor "1" → "1.1" (see below).

ExplicitImports findings (6 checks)

Check	Result
no_implicit_imports	PASS
no_stale_explicit_imports	PASS
all_explicit_imports_via_owners	PASS
all_qualified_accesses_via_owners	PASS
all_explicit_imports_are_public	PASS (with ignores)
all_qualified_accesses_are_public	PASS (with ignores)

The two public-API checks ignore upstream non-public names that CurveFit qualifies/imports, documented inline by source package — these go public as the base libraries declare their APIs:

• SciMLBase: AbstractLinearAlgorithm, AbstractNonlinearAlgorithm, successful_retcode; SciMLBase.ReturnCode: Success, T
• NonlinearSolveBase: AutoSpecializeCallable, NonlinearSolvePolyAlgorithmCache, Utils, get_fu; NonlinearSolveBase.Utils: clean_sprint_struct
• StatsAPI: coef, confint, dof, dof_residual, fitted, nobs, predict, residuals, rss, stderror, vcov
• CommonSolve: init, solve, solve!
• Base: rtoldefault

No own (CurveFit) non-public names needed ignoring. 0 hard FAILs, 0 known-broken.

Coverage note: the old test/explicit_imports.jl also ran check_no_self_qualified_accesses (passing), which is not part of the standardized run_qa 6-check set, so it is not carried over.

PrecompileTools floor bump (Downgrade fix)

CurveFit explicit-imports only the PrecompileTools macros (using PrecompileTools: @compile_workload, @setup_workload). On PrecompileTools 1.0.0, the macro expansion references the bare PrecompileTools module name, which is undefined under a macro-only import → UndefVarError: PrecompileTools not defined on the Downgrade(lts) lane. Verified locally: breaks on 1.0.0, clean on 1.1.0. Floor raised to "1.1".

Verification (local, Julia 1.10, released SciMLTesting 1.6.0)

• QA via run_tests() folder model (GROUP=QA): Quality Assurance | 17 17 — Aqua 11 (Method ambiguity, Unbound type params, Undefined exports, Project.toml compare, Stale deps, Compat bounds ×4, Piracy, Persistent tasks), ExplicitImports 6. 0 fail/error/broken.
• Core via Pkg.test() (GROUP=Core): all pass, Testing CurveFit tests passed (exit 0) — root env resolves without ExplicitImports.

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