Add multiple transitions from one source stage — split

Convenience sugar for branching stage fan-out. Compiles into deterministic add_transition() calls.

Usage

split_stage(
  spec,
  from,
  into,
  priority_start = 1L,
  consume = TRUE,
  allow_overlap = FALSE,
  seeding = "by_source_rank"
)

spec: A tournament_spec object.
from: Source stage ID, or from_previous().
into: Named list mapping destination stage IDs to transition rules, or branch configs with a required rule field.
priority_start: Starting priority for branch transitions when a branch does not explicitly provide priority.
consume: Default consume value for branches.
allow_overlap: Default allow_overlap value for branches.
seeding: Default seeding policy for branches.

Updated tournament_spec.