medsim 0.5.1 — fixed-branch ARIV

New features

• medsim_method_mbco_mi() gains ariv = c("own", "fixed") and always emits
  additive branch diagnostics alongside the method contract: indirect_p_fixed
  (D4 p-value with the ARIV r4 recomputed on the branch the stacked
  constrained fit selected), branch_mix (imputations disagree on the
  constrained branch), p_branch_a, stacked_branch, r4, r4_fixed.
  .medsim_d4_mbco() gains fixed_branch = and returns stacked_branch.
  Motivation: the standard Chan–Meng r4 averages per-imputation MBCO
  statistics each computed on its own winning branch, so branch disagreement
  pulls the average down and under-estimates the ARIV (Missing Effect
  comparator pilot, 2026-08-16). Default ariv = "own" keeps legacy behavior;
  indirect_p is unchanged unless ariv = "fixed".
  The stacked D4 model is fit once per replication and shared by both ARIV
  computations (#44).

Bug fixes

• Gate A.2 (seed-collapse audit in medsim_combine_chunks() /
  medsim_audit_results()) no longer flags the medsim_method_mbco_mi()
  branch diagnostics: branch_mix, stacked_branch, p_branch_a, r4,
  r4_fixed join converged/branch_switch in the default
  collapse_exclude (they are legitimately low-cardinality — 0/1 flags, an
  m-valued share, ARIVs clamped at 0). Without this, a chunked MBCO-MI run
  raised [collapse] violations and the default on_violation = "stop"
  refused to combine it (pre-release adversarial review, #44 follow-up).