refactor: move optional arguments of layout 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 15 exported layout 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/layout.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
layout_as_bipartite()	graph, types	hgap, vgap, maxiter
layout_as_star()	graph	center, order
layout_as_tree()	graph	root, circular, rootlevel, mode, flip.y
layout_on_grid()	graph	width, height, dim
layout_randomly()	graph	dim
layout_with_dh()	graph	coords, maxiter, fineiter, cool.fact, weight.node.dist, weight.border, weight.edge.lengths, weight.edge.crossings, weight.node.edge.dist
layout_with_drl()	graph	use.seed, seed, options, weights, dim
layout_with_fr()	graph	coords, dim, niter, start.temp, grid, weights, minx, maxx, miny, maxy, minz, maxz, coolexp, maxdelta, area, repulserad, maxiter
layout_with_gem()	graph	coords, maxiter, temp.max, temp.min, temp.init
layout_with_graphopt()	graph	start, niter, charge, mass, spring.length, spring.constant, max.sa.movement
layout_with_kk()	graph	coords, dim, maxiter, epsilon, kkconst, weights, minx, maxx, miny, maxy, minz, maxz, niter, sigma, initemp, coolexp, start
layout_with_lgl()	graph	maxiter, maxdelta, area, coolexp, repulserad, cellsize, root
layout_with_sugiyama()	graph	layers, hgap, vgap, maxiter, weights, attributes
merge_coords()	graphs, layouts	method
norm_coords()	layout	xmin, xmax, ymin, ymax, zmin, zmax

Placement rethink: component_wise() and normalize() were originally
migrated here with an empty head (all arguments keyword-only) — such
signatures are now disallowed (the head must never be empty; see the updated
rule in #2757), so both were dropped from this PR and keep their original
signatures.

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 abdaf1b is merged into main:

• ✔️as_adjacency_matrix: 776ms -> 779ms [-0.89%, +1.75%]
• ✔️as_biadjacency_matrix: 748ms -> 748ms [-0.5%, +0.6%]
• ✔️as_data_frame_both: 1.5ms -> 1.48ms [-2.76%, +1.01%]
• ✔️as_long_data_frame: 3.92ms -> 3.92ms [-1.4%, +1.5%]
• ✔️es_attr_filter: 2.7ms -> 2.71ms [-1.24%, +1.97%]
• 🚀graph_from_adjacency_matrix: 121ms -> 120ms [-2.24%, -0.15%]
• ✔️graph_from_data_frame: 3.35ms -> 3.38ms [-0.91%, +2.75%]
• ✔️vs_attr_filter: 1.53ms -> 1.53ms [-2.61%, +2.6%]
• ✔️vs_by_name: 973µs -> 985µs [-1.11%, +3.46%]
  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 2669762 is merged into main:

• ✔️as_adjacency_matrix: 684ms -> 685ms [-1.51%, +1.61%]
• ✔️as_biadjacency_matrix: 646ms -> 647ms [-1.29%, +1.57%]
• ✔️as_data_frame_both: 1.28ms -> 1.27ms [-3.32%, +2.35%]
• ✔️as_long_data_frame: 3.18ms -> 3.17ms [-2.33%, +1.91%]
• ✔️es_attr_filter: 2.22ms -> 2.26ms [-1.31%, +4.86%]
• ✔️graph_from_adjacency_matrix: 132ms -> 132ms [-1.36%, +0.91%]
• ✔️graph_from_data_frame: 2.85ms -> 2.84ms [-2.66%, +1.95%]
• ✔️vs_attr_filter: 1.27ms -> 1.28ms [-3.07%, +3.59%]
• ✔️vs_by_name: 862µs -> 880µs [-2.2%, +6.3%]
  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 8eef478 is merged into main:

• ✔️as_adjacency_matrix: 945ms -> 941ms [-1.51%, +0.85%]
• ✔️as_biadjacency_matrix: 876ms -> 883ms [-0.6%, +2.06%]
• ✔️as_data_frame_both: 1.8ms -> 1.79ms [-2.61%, +1.2%]
• ✔️as_long_data_frame: 4.48ms -> 4.47ms [-2.21%, +1.57%]
• ✔️es_attr_filter: 3.07ms -> 3.08ms [-3.34%, +4.22%]
• ✔️graph_from_adjacency_matrix: 166ms -> 168ms [-2.4%, +4.8%]
• ✔️graph_from_data_frame: 3.8ms -> 3.76ms [-2.8%, +0.68%]
• ✔️vs_attr_filter: 1.78ms -> 1.8ms [-2.24%, +4.38%]
• ✔️vs_by_name: 1.19ms -> 1.17ms [-3.6%, +1.28%]
  Further explanation regarding interpretation and methodology can be found in the documentation.