refactor: replace .Call() with _impl() functions (subset of #2562)

[Copilot]

Summary

This PR extracts the actual .Call() → _impl() code replacements from #2562, without the comment-only additions. It is a strict subset of the diff of that PR, intended to be a commit that can be merged into it.

Replacements Made (.Call() → _impl())

Function	Replacement
cluster_spinglass()	community_spinglass_impl() / community_spinglass_single_impl()
subcomponent()	subcomponent_impl()
girth()	girth_impl()
ego_size()	neighborhood_size_impl()
ego()	neighborhood_impl()
make_ego_graph()	neighborhood_graphs_impl()
ivs()	independent_vertex_sets_impl()
community.to.membership2()	community_to_membership_impl()
count_components()	connected_components_impl(details=TRUE)$no
count_max_cliques()	maximal_cliques_count_impl() / maximal_cliques_subset_impl()
bipartite_projection()	bipartite_projection_impl()
disjoint_union()	disjoint_union_many_impl()
add_edges()	add_edges_impl()
read.graph.edgelist()	read_graph_edgelist_impl()
read.graph.lgl()	read_graph_lgl_impl()
write.graph.dimacs()	write_graph_dimacs_flow_impl()
Layout functions	Various layout_*_impl() functions

Also adds missing on.exit(.Call(Rx_igraph_finalizer)) in identical_graphs() and read.graph.dimacs().

Tests

All 7994 tests pass.

• Fixes Replace all remaining .Call() expressions with calls to _impl functions #2561 (partial)