`Pkg.test()` aborts in precompile: GraphViz extension fails to initialize on Julia 1.12 (`libgvc` UndefVarError)

[docxology]

Summary

Pkg.test() aborts during precompilation because the weak-dep extension
GraphPPLGraphVizExt fails to load. The error originates inside GraphViz.jl's own
__init__:

InitError: UndefVarError: `libgvc` not defined in `GraphViz`
  [2] __init__() ... GraphViz.jl:206
  [3] top-level scope ... GraphPPLGraphVizExt.jl:2

This is an upstream GraphViz.jl / Julia-1.12 platform incompatibility (libgvc loading),
pulled in via the test target (Project.toml [targets] test = [..., "GraphViz", ...]),
not a defect in GraphPPL's inference logic. Consequence: the CI gate cannot be reproduced
locally on 1.12, and the extension's own behaviour is never exercised.

(On the audit machine, Pkg.precompile() succeeds for the core package; Pkg.test() fails
at the extension. Core @testitem files run fine when the extension is not loaded.)

Suggested fix

• Preferable: pin GraphViz to a version that initialises on Julia 1.12, or ensure the
  GraphViz jll/libgvc is present.
• Resilient: guard the extension's top-level GraphViz.Context creation in a
  try/catch with a @warn so a broken optional dependency degrades instead of aborting
  the whole test run.