refactor: move optional arguments of centrality 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 10 exported centrality 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/centrality.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
alpha_centrality()	graph, nodes	alpha, loops, exo, weights, tol, sparse
betweenness()	graph, v	directed, weights, normalized, cutoff
closeness()	graph, vids	mode, weights, normalized, cutoff
diversity()	graph	weights, vids
edge_betweenness()	graph, e	directed, weights, cutoff
harmonic_centrality()	graph, vids	mode, weights, normalized, cutoff
page_rank()	graph	algo, vids, directed, damping, personalized, weights, options
power_centrality()	graph, nodes	loops, exponent, rescale, tol, sparse, weights
strength()	graph, vids	mode, loops, weights
subgraph_centrality()	graph	diag

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 e36651f is merged into main:

• ✔️as_adjacency_matrix: 768ms -> 775ms [-0.16%, +1.85%]
• ✔️as_biadjacency_matrix: 773ms -> 775ms [-0.93%, +1.56%]
• ✔️as_data_frame_both: 1.56ms -> 1.58ms [-1.21%, +4.25%]
• ✔️as_long_data_frame: 4.01ms -> 4.03ms [-1.53%, +2.39%]
• ✔️es_attr_filter: 2.87ms -> 2.85ms [-8.26%, +6.32%]
• ✔️graph_from_adjacency_matrix: 120ms -> 120ms [-0.83%, +1.37%]
• ✔️graph_from_data_frame: 3.42ms -> 3.41ms [-1.7%, +1.46%]
• ✔️vs_attr_filter: 1.65ms -> 1.62ms [-3.81%, +0.58%]
• ✔️vs_by_name: 1.03ms -> 1.02ms [-3.2%, +1.82%]
  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 561954e is merged into main:

• ✔️as_adjacency_matrix: 886ms -> 885ms [-1.32%, +0.95%]
• ✔️as_biadjacency_matrix: 855ms -> 852ms [-1.79%, +1.04%]
• ✔️as_data_frame_both: 1.76ms -> 1.74ms [-2.42%, +0.37%]
• ✔️as_long_data_frame: 4.61ms -> 4.6ms [-2.06%, +1.9%]
• ✔️es_attr_filter: 3.19ms -> 3.12ms [-4.65%, +0.59%]
• ✔️graph_from_adjacency_matrix: 133ms -> 133ms [-1.58%, +1.33%]
• ✔️graph_from_data_frame: 3.74ms -> 3.74ms [-1.79%, +1.9%]
• ✔️vs_attr_filter: 1.82ms -> 1.77ms [-5.89%, +0.84%]
• ✔️vs_by_name: 1.19ms -> 1.19ms [-1.81%, +2.7%]
  Further explanation regarding interpretation and methodology can be found in the documentation.