refactor: move optional arguments of flow 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 flow 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/flow.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
cohesive_blocks()	graph	labels
export_pajek()	blocks, graph, file	project.file
adhesion()	graph	checks
dominator_tree()	graph, root	mode
edge_connectivity()	graph, source, target	checks
max_flow()	graph, source, target	capacity
min_cut()	graph, source, target	capacity, value.only
st_min_cuts()	graph, source, target	capacity
vertex_connectivity()	graph, source, target	checks

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 87bd356 is merged into main:

• ✔️as_adjacency_matrix: 882ms -> 879ms [-1.26%, +0.58%]
• ✔️as_biadjacency_matrix: 841ms -> 839ms [-0.78%, +0.36%]
• ✔️as_data_frame_both: 1.71ms -> 1.72ms [-1.49%, +1.87%]
• ✔️as_long_data_frame: 4.42ms -> 4.53ms [-2.09%, +7.05%]
• ❗🐌es_attr_filter: 3.17ms -> 3.25ms [+0.07%, +4.96%]
• ✔️graph_from_adjacency_matrix: 135ms -> 137ms [-0.34%, +2.91%]
• ✔️graph_from_data_frame: 3.68ms -> 3.66ms [-2.06%, +1.08%]
• ✔️vs_attr_filter: 1.8ms -> 1.81ms [-1.56%, +2.26%]
• ✔️vs_by_name: 1.2ms -> 1.18ms [-3.03%, +1.19%]
  Further explanation regarding interpretation and methodology can be found in the documentation.