A judge in Rhode Island ruled Tuesday that Steward Health Care Systems LLC of Boston can move forward with negotiating a takeover of financially troubled Landmark Medical Center in Woonsocket, which has been in court-appointed receivership for the past three years. Landmark had begun talking about a sale to Steward's predecessor, Caritas Christi Health Care, in 2009, but the talks broke off in December without an agreement. The parties recently resumed negotiations and were working yesterday to finalize the terms of a buyout agreement that could be unveiled today. If a deal is reached, it would need Rhode Island regulators' approval and represent Steward's first out-of-state acquisition.