Fix spec gaps

[shmercer]

What changed

• Added new controller/config flag allow_spoke_spoke_cross_set (default FALSE) in R/adaptive_state.R.

  • Exposed the new flag in adaptive config docs:
    • R/adaptive_run.R
    • R/adaptive_rank.R
    • Regenerated docs: man/adaptive_rank_start.Rd, man/adaptive_rank_run_live.Rd, man/adaptive_rank.Rd
  • Implemented Phase B candidate branching in R/adaptive_round_candidates.R:
    • Default path unchanged: hub↔spoke only.
    • Enabled path: allows selected-spoke↔other-spoke cross-set candidates (still forbids within-set edges).
  • Updated linking predictive utility to support candidate endpoint sets beyond hub+single-spoke in R/
    adaptive_select.R.
  • Updated step logging attribution in R/adaptive_step.R so cross-set rows retain link_spoke_id for selected spoke
    even on non-hub cross-set edges.
  • Tightened link_stop_eligible semantics in R/adaptive_btl_refit.R to require required gate eligibility/definedness
    (not just lag+diagnostics presence).

  Tests added/updated

  • tests/testthat/test-5049-linking-candidates-round-routing.R
    • Added enabled-policy spoke↔spoke candidate test.
    • Kept default hub↔spoke behavior test.
  • tests/testthat/test-5050-linking-refit-transforms.R
    • Added case where missing required gate definition forces link_stop_eligible = FALSE.
  • tests/testthat/test-5045-adaptive-helper-branches.R
    • Added config validation coverage for new flag (invalid type + valid TRUE branch).