`return.vs.es`: fix performance, then deprecate and remove

[schochastics]

This issue unifies three related threads:

• Remove return.vs.es option #1652 — remove the return.vs.es option.
• Some functions that return vertex or edge sequences ignore the value of the option return.vs.es #1614 — some functions ignore return.vs.es (consistency); partially
  addressed by PR [WIP] Fix functions ignoring return.vs.es option #2439.
• Slower speed vs basic edgelist in some tasks #557 — igraph.vs/igraph.es are much slower than plain edge lists/indices.

They are the same problem from three angles: the option exists mainly for
performance (#557), it is applied inconsistently (#1614), and the
long-term goal is to remove it (#1652) with as.integer() as the
replacement. Removal is blocked until the performance gap is closed — e.g.
max_cliques(sample_gnp(500, 0.15)) is ~8× slower with return.vs.es = TRUE
(268 ms vs 33 ms) because create_vs() builds names eagerly. The endorsed
direction is lazy (ALTREP) naming so igraph.vs/igraph.es is as cheap as a
numeric vector and names are materialized only on print/use.

Steps to close

Phase 0 — Close the performance gap (prerequisite)

• Make vertex/edge sequence construction lazy (ALTREP-backed names), so
  create_vs()/create_es() and E() defer the names/vnames work
  until printed or accessed.
• Benchmark gate: max_cliques(sample_gnp(500, 0.15)) with the default
  approaches the old return.vs.es = FALSE timing. If the gap isn't
  closed, stop here — do not proceed to removal.

Phase 1 — Decouple internal usage from the public option

• Give the _impl layer a private numeric-output path that does not read
  return.vs.es.
• Replace the with_igraph_opt(list(return.vs.es = FALSE), ...) wrappers in
  all_simple_paths() and get_edge_ids() with that mechanism (no
  user-visible change).

Phase 2 — Soft-deprecate the option

• Emit lifecycle::deprecate_warn() when a user sets return.vs.es,
  pointing to as.integer() / as_ids().
• Update the option's roxygen doc entry with a deprecation badge + recipe.
• Add a NEWS entry. Keep the option functional for at least one release.

Phase 3 — Remove

• Drop the if (igraph_opt("return.vs.es")) guard from the OUTCONV
  templates in tools/stimulus/types-RR.yaml (keep the create_vs/
  create_es body; keep the vcount() != 0 guard for VERTEX_ROOT).
• Regenerate the R/aaa-*.R files (103 sites) via codegen — do not hand-edit.
• Remove the 24 hand-written guards (keep their wrapping bodies).
• Remove return.vs.es from .igraph.pars and its doc entry in R/par.R;
  make the deprecated setter hard-error or no-op.
• Update roxygen prose referencing the option (regenerate .Rd in CI).
• Update/trim tests; full R CMD check; run tools/revdep.R before release.