feat: Add count_loops() to R interface

[Copilot]

Exposes the existing C function igraph_count_loops() to the R interface as count_loops().

Changes

• R/structural-properties.R: Added count_loops() function that returns the total number of self-loop edges as a numeric scalar
• Documentation: Integrated into existing which_multiple help page alongside any_loop() and which_loop()
• Tests: Added test coverage for graphs with loops, without loops, and edge cases

Usage

library(igraph)

# Graph with 3 self-loops
g <- make_graph(c(1, 1, 2, 2, 3, 3, 4, 5))
count_loops(g)  # Returns 3

# Complements existing loop detection functions
any_loop(g)     # Returns TRUE
which_loop(g)   # Returns c(TRUE, TRUE, TRUE, FALSE)

The function follows the established pattern of count_multiple() and integrates with the existing family of loop/multiple edge detection functions.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add igraph_count_loops() to R</issue_title>
<issue_description>Add igraph_count_loops() to R. I suggest count_loops() as the name. I would consider this low priority.</issue_description>

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

• Fixes Add igraph_count_loops() to R #1379

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[schochastics]

LGTM