refactor: move optional arguments of community functions behind the ellipsis

[krlmlr]

Part of the repo-wide ellipsis migration coordinated in #2757 —
see that PR for the full rationale (CONTRIBUTING.md, Argument Order and the Ellipsis).
Based directly on main (the registry split landed via #2779); a single topic commit.

What this does

Inserts ... between the defining arguments (head) and the optional
modifiers (tail) of 9 exported community functions.
Arguments after ... become keyword-only.

• Legacy positional or abbreviated calls are recovered by the generated
  ARG_HANDLE block and emit a single lifecycle::deprecate_soft("3.0.0", …) —
  behavior is unchanged.
• No defaults change, no arguments are renamed, deprecated functions untouched.
• Registry: tools/migrations/community.R; blocks regenerated via
  Rscript tools/generate-migrations.R (idempotent, CI-checked).
• Rd usage/arguments updated mechanically (see note below).

New signatures

function	head (positional)	keyword-only tail
cluster_edge_betweenness()	graph	weights, directed, edge.betweenness, merges, bridges, modularity, membership
cluster_fast_greedy()	graph	merges, modularity, membership, weights
cluster_infomap()	graph	e.weights, v.weights, nb.trials, modularity
cluster_louvain()	graph	weights, resolution
cluster_optimal()	graph	weights
cluster_spinglass()	graph	weights, vertex, spins, parupdate, start.temp, stop.temp, cool.fact, update.rule, gamma, implementation, gamma.minus
cluster_walktrap()	graph	weights, steps, merges, modularity, membership
make_clusters()	graph, membership	algorithm, merges, modularity
modularity_matrix()	graph, membership	weights, resolution, directed

Notes for review

• The environment used to prepare this PR cannot install igraph.r2cdocs
  (GitHub API unreachable), so man/*.Rd files were updated mechanically
  (usage + ... argument item) instead of via devtools::document().
  R CMD check's usage↔formals validation passes; a follow-up
  devtools::document() run may reflow whitespace but should produce no
  semantic diff.
• Package tests that called these functions positionally were updated to
  named arguments (they are the same soft-deprecation user code will see).

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if 93936d7 is merged into main:

• ✔️as_adjacency_matrix: 828ms -> 827ms [-1.11%, +1.07%]
• ✔️as_biadjacency_matrix: 793ms -> 792ms [-0.78%, +0.55%]
• ✔️as_data_frame_both: 1.61ms -> 1.62ms [-1.57%, +3.04%]
• ✔️as_long_data_frame: 4.18ms -> 4.19ms [-2.87%, +3.41%]
• ✔️es_attr_filter: 2.96ms -> 2.88ms [-6%, +0.18%]
• ✔️graph_from_adjacency_matrix: 126ms -> 125ms [-1.49%, +0.71%]
• ✔️graph_from_data_frame: 3.53ms -> 3.51ms [-2.13%, +0.99%]
• ✔️vs_attr_filter: 1.68ms -> 1.67ms [-3.06%, +1.57%]
• ✔️vs_by_name: 1.07ms -> 1.07ms [-2.78%, +2.84%]
  Further explanation regarding interpretation and methodology can be found in the documentation.

[github-actions]

This is how benchmark results would change (along with a 95% confidence interval in relative change) if d9a5291 is merged into main:

• ❗🐌as_adjacency_matrix: 783ms -> 790ms [+0.08%, +1.67%]
• ✔️as_biadjacency_matrix: 760ms -> 756ms [-1.38%, +0.36%]
• ✔️as_data_frame_both: 1.61ms -> 1.56ms [-9.92%, +4.29%]
• ❗🐌as_long_data_frame: 3.91ms -> 3.97ms [+0.28%, +2.64%]
• ✔️es_attr_filter: 2.82ms -> 2.75ms [-6.77%, +2.28%]
• ✔️graph_from_adjacency_matrix: 123ms -> 122ms [-1.55%, +0.25%]
• ✔️graph_from_data_frame: 3.39ms -> 3.37ms [-1.74%, +0.62%]
• ✔️vs_attr_filter: 1.58ms -> 1.56ms [-3.22%, +0.88%]
• ✔️vs_by_name: 1ms -> 1.02ms [-0.83%, +4.8%]
  Further explanation regarding interpretation and methodology can be found in the documentation.