compat: allow GraphViz 0.3

[bvdmitri]

Change

Project.toml [compat]: GraphViz = "0.2" → GraphViz = "0.2, 0.3".

GraphViz 0.3.0 (Graphviz_jll v15) is the current release; we were pinned to the 0.2 series.

Verification

Checked on Julia 1.10, 1.11 and 1.12 that with this change GraphViz 0.3.0 is the version
selected, the GraphPPLGraphVizExt extension precompiles and loads, GraphViz.Graph(::String)
constructs, and SVG rendering works. GraphViz.load is still an extensible generic (2 methods),
so ext/GraphPPLGraphVizExt.jl:520 needs no changes.

Full test suite on Julia 1.12 with 0.3.0: 258/258 test items, 76803 pass, 1 broken — including
the ext/graphviz_integration_tests.jl item, which exercises the extension against the whole
model zoo.

Relationship to #313

This is a compat bump, not a fix for #313, and it does not close it.

#313 reports InitError: UndefVarError: libgvc not defined in GraphViz during Pkg.test() on
Julia 1.12. That could not be reproduced here: GraphViz 0.2.0, 0.2.1 and 0.3.0 all initialize
correctly on Julia 1.12 (macOS arm64), and the extension loads in every case. The reported
failure looks environment-specific — most likely a partial or corrupted Graphviz_jll artifact
on the reporting machine rather than a version incompatibility.

Landing this on its own merits: staying on 0.2 blocks users from a maintained release for no
reason.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.72%. Comparing base (4cc7c6c) to head (6953eb5).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #322   +/-   ##
=======================================
  Coverage   90.72%   90.72%           
=======================================
  Files          16       16           
  Lines        2263     2263           
=======================================
  Hits         2053     2053           
  Misses        210      210           

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