fix: use canonical structure() attribute names

[krlmlr]

LLM disclosure: this PR was prepared with Claude Code (Claude Opus 4.8).

• By submitting this pull request, I assign the copyright of my contribution to The igraph development team.

What

R-devel deprecates the special structure() arguments .Names, .Dim and .Dimnames; they emit warnings that the test suite escalates (the ~27 warnings noted on #2749).

Replace them with the canonical names, dim and dimnames. structure() maps both spellings to the same attributes, so the constructed objects are identical — no behavior change.

• 42 occurrences: 9 tests/testthat/*.R fixtures (mostly dput() output) and one R/plot.common.R fixture.
• Each replacement verified to be a structure() attribute argument — the list(...) data and its inner element names are untouched.
• All edited files parse cleanly.

Scope

Code changes only, branched off main. The CI/workflow side of the R-devel work lives separately:

• ci: disable Bioconductor on older R so pak can resolve dependencies #2753 — disable Bioconductor on old R (via pak's PKG_USE_BIOCONDUCTOR) so pak resolution stops failing on R 4.2/4.3 where the archived graph releases now 404.
• ci: run R-CMD-check on claude/** branches #2754 — run R-CMD-check on claude/** branches.
• fix: declare R_getRegisteredNamespace() so the C23 sanitizer build compiles #2749 — the R_getRegisteredNamespace C23 sanitizer error, which is a stale CSAN Docker image, not a package bug.

🤖 Generated with Claude Code

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Generated by Claude Code

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 8ced78f is merged into main:

• ✔️as_adjacency_matrix: 788ms -> 785ms [-2.13%, +1.19%]
• ✔️as_biadjacency_matrix: 755ms -> 752ms [-2.22%, +1.63%]
• ✔️as_data_frame_both: 1.43ms -> 1.47ms [-2.23%, +7.94%]
• ✔️as_long_data_frame: 3.52ms -> 3.52ms [-4%, +4%]
• ✔️es_attr_filter: 2.68ms -> 2.71ms [-3.36%, +6.12%]
• ✔️graph_from_adjacency_matrix: 148ms -> 147ms [-2.42%, +0.87%]
• ❗🐌graph_from_data_frame: 3.06ms -> 3.13ms [+0.03%, +4.78%]
• ✔️vs_attr_filter: 1.49ms -> 1.5ms [-3.81%, +4.24%]
• ✔️vs_by_name: 997µs -> 1ms [-4.01%, +4.72%]
  Further explanation regarding interpretation and methodology can be found in the documentation.