Fix CI: update vdiffr snapshots, fix Rd docs, add Copilot setup

[Copilot]

• Investigate CI failures (vdiffr snapshot tests failing in test-hgraph.R + documentation mismatch)
• Generate updated SVG snapshots for hGraph tests using geom_curve output (with transition boxes)
• Fix documentation mismatch in man/hGraph.Rd (add curvedflag and curvature parameters)
• Add copilot-setup-steps.yml workflow (without Rust step)
• Add .github/copilot-instructions.md with R CMD check instructions
• Remove redundant curvedflag param; use curvature == 0 for straight lines (reviewer feedback)
• Fix operator precedence bug: if/else inside ggplot + chain was silently dropping geom_rect and geom_text layers when curvedflag=TRUE (transition boxes were missing from all snapshots)
• Regenerate snapshots with transition boxes now present
• Update copilot-instructions.md: use R CMD check CLI, use roxygen2::roxygenize() for docs/NAMESPACE, merge documentation+NAMESPACE bullets

Original prompt

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This section details on the original issue you should resolve

<issue_title>Replace geom_line() with geom_curve()</issue_title>
<issue_description>Idea from @keaven </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Replace geom_line() with geom_curve() #43

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[yihui]

why were transition boxes lost in the new svg snapshots?

[yihui]

dont' use any packages; simply run command line R CMD check

[Copilot]

Updated in ec59bf9 to use R CMD build . && R CMD check --no-manual --as-cran *.tar.gz.

[yihui]

that's not the way to update documentation: run roxygen2::roxygenize() instead

[Copilot]

Updated in ec59bf9 to use roxygen2::roxygenize().

[yihui]

merge this item with the documentation item: in general, always run roxygen2 after making changes in source code to update documentation, namespace, etc.

[Copilot]

Merged into a single "Documentation and NAMESPACE" bullet in ec59bf9.

[yihui]

no need to add two args: curvature alone would be enough---curvature == 0 means segments

[Copilot]

Removed curvedflag in ec59bf9. Now curvature = 0 draws straight segments (geom_segment), and any non-zero value draws curves (geom_curve).

[Copilot]

why were transition boxes lost in the new svg snapshots?

There was an operator precedence bug in R: if (curvedflag) { geom_curve(...) } else { geom_segment(...) } + geom_rect(...) + geom_text(...) is parsed as if (curvedflag) { geom_curve(...) } else { geom_segment(...) + geom_rect(...) + geom_text(...) } — so geom_rect and geom_text were only added to the else branch, and were silently dropped when curvedflag=TRUE. Fixed in ec59bf9 by assigning the conditional layer to a variable first (arrowLayer <- if (...) ...) and then adding it to the chain separately. The snapshots now include the transition boxes.