fix: cut_at() handles partial dendrograms from cluster_leading_eigen()

[Copilot]

cut_at() incorrectly used igraph_community_to_membership for all hierarchical community structures. cluster_leading_eigen() produces a partial dendrogram where merges entries are community IDs (not vertex IDs), requiring the dedicated igraph_le_community_to_membership function.

Changes

• R/community.R — cut_at(): Detects leading eigenvector results via communities$algorithm == "leading eigenvector" and routes to le_community_to_membership_impl() with correct 0-based index conversion. Handles boundary warnings for out-of-range no and steps values.
• tests/testthat/test-community.R: New test block covering no= and steps= variants, including warning behavior for out-of-range inputs.

Example

g <- make_full_graph(5) %du% make_full_graph(5) %du% make_full_graph(5)
g <- add_edges(g, c(1, 6, 1, 11, 6, 11))
lec <- cluster_leading_eigen(g)  # partial dendrogram: 3 communities, 2 merges

cut_at(lec, no = 3)  # previously returned garbage; now returns original 3-community membership
cut_at(lec, no = 2)  # merge 2 of 3 communities
cut_at(lec, no = 1)  # all vertices in one community

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

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.