fix(sessions): turn-advance coordination (concurrent-add race)

[mads-jm]

advanceTurn() can fire twice on concurrent adds; UI distrusts stored currentTurnUserId and re-derives from track history (TurnManagementPanel.tsx:127). Coordinate turn advancement across peers.

Plan & context: WhatNext - docs/06 reports/report-260627-issue-reconciliation.md and report-260627-mvp-state-of-the-union.md.