Support global sparsity tracing through distribution constructors

[devmotion]

Motivation

@check_args validates a distribution's constructor arguments with comparisons such as σ ≥ 0. Under SparseConnectivityTracer's global TracerSparsityDetector, those comparisons run on primal-free tracers and return a tracer rather than a Bool, so they cannot be used in the boolean context the checks require. As a result, the global sparsity pattern cannot currently be traced through any distribution construction — jacobian_sparsity(x -> mean(Normal(x[1], x[2])), x, TracerSparsityDetector()) errors inside check_args.

Change

Introduce an overloadable per-argument predicate _arg_checkable(x) (default true). @check_args now forwards the explicitly-named checked arguments to check_args, which skips the checks when any argument is not checkable:

_arg_checkable(@nospecialize(_)) = true

function check_args(f::F, check::Bool, args...) where {F}
    (check && all(_arg_checkable, args)) && f()
    nothing
end

A new weak-dependency extension DistributionsSparseConnectivityTracerExt provides the single overload:

Distributions._arg_checkable(::AbstractTracer) = false

AbstractTracer covers exactly the primal-free global tracers (GradientTracer/HessianTracer). Local sparsity detection (TracerLocalSparsityDetector) passes primal-carrying Duals, which are <: Real (not <: AbstractTracer), so their comparisons evaluate to Bool as usual and the checks run unchanged — invalid arguments are still rejected under local tracing.

Ordinary numeric construction is unaffected (args are plain numbers, _arg_checkable is true).

Tests

test/sparseconnectivitytracer.jl checks that global gradient and (block-off-diagonal) Hessian sparsity trace through Normal construction, and that local detection still enforces argument validity.

---

This pull request was prepared with the assistance of generative AI (Claude Code).

[codecov-commenter]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.65%. Comparing base (a7ed7b6) to head (17a40a1).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/utils.jl	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2064      +/-   ##
==========================================
- Coverage   86.65%   86.65%   -0.01%     
==========================================
  Files         148      149       +1     
  Lines        8919     8925       +6     
==========================================
+ Hits         7729     7734       +5     
- Misses       1190     1191       +1     

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[andreasnoack]

Looks good. Based on Claude's review assistance, I've filed #2067 to keep track of possible follow-up work needed to support all distributions.