refactor: move optional arguments of plotting 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 7 exported plotting 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/plotting.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
igraph_opt()	x	default
curve_multiple()	graph	start
add_shape()	shape	clip, plot, parameters
tk_close()	tkp.id	window.close
tk_coords()	tkp.id	norm
tk_fit()	tkp.id	width, height
tk_rotate()	tkp.id	degree, rad

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 ff39dd5 is merged into main:

• ✔️as_adjacency_matrix: 862ms -> 864ms [-0.79%, +1.34%]
• ✔️as_biadjacency_matrix: 827ms -> 828ms [-1%, +1.17%]
• ✔️as_data_frame_both: 1.75ms -> 1.73ms [-3.52%, +0.65%]
• ✔️as_long_data_frame: 4.48ms -> 4.4ms [-4.52%, +0.87%]
• ✔️es_attr_filter: 3.01ms -> 3.02ms [-2.54%, +2.68%]
• ✔️graph_from_adjacency_matrix: 132ms -> 132ms [-2.74%, +2.23%]
• ✔️graph_from_data_frame: 3.64ms -> 3.67ms [-0.43%, +2.07%]
• ✔️vs_attr_filter: 1.71ms -> 1.69ms [-2.62%, +1.1%]
• ✔️vs_by_name: 1.12ms -> 1.11ms [-3.06%, +1.58%]
  Further explanation regarding interpretation and methodology can be found in the documentation.