[krlmlr]

This adds tests that pin what main computes today wherever the upcoming igraph C core 1.0 changes the result. Each test passes with the vendored C core 0.10.17 and fails with 1.0.1+, so when the vendoring series is rebuilt from main, the vendor commit that changes a behaviour has to flip its expectation, and every user-visible change shows up in the review diff. It is stacked on #2996.

What it pins

• hits_scores() on an edgeless graph returns NaN without a warning, and on an undirected graph it does not warn and returns the expected value (test-centrality.R).
• eigen_centrality() on two disconnected graphs does not warn and returns the expected vectors (test-centrality.R).
• The cluster_infomap() code length on the karate graph (test-community.R).
• The edge order of graph_from_adjacency_matrix() for directed input (test-adjacency.R) and of graph_from_biadjacency_matrix() for dense unweighted input (test-incidence.R).
• The random streams of seeded sample_islands() (test-games.R), layout_with_gem() and layout_with_dh() (test-layout.R).

The ARPACK error wording is already pinned by an existing snapshot, so it needs no new test.

Also included

Commit dcfe7b3 from claude/f-nits (draft #2992) makes the eigen_centrality() stress test in test-centrality.R expect the "eigenvector centralities are nearly zero" warning and muffle only that one. The C core 1.0 emits it for most of the disconnected random graphs in that test, which printed hundreds of warnings on a passing test. The engine on main never emits it, so here the handler never fires and the test is unchanged in effect.

Checks

devtools::document() and air format change nothing. The six touched test files pass against a build of this branch: 241 blocks, 0 failures, 0 warnings, and the one existing skip (pcalg not installed).

LLM use: this PR was prepared with Claude Code (Claude Opus 5.5), at the maintainer's request. The pinning tests come from the subsession https://claude.ai/code/session_016jy2KuQGnR2SqMEUJ3dx35, and the stress-test commit from https://claude.ai/code/session_01L5n6BLiYoPmk3NFiYDSKZZ.

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

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