LONDON — Kim Leadbeater has a foot-high sheaf of paper marked “PMB” on her Westminster office desk. The Labour MP’s private member’s bill to legalize assisted dying in England and Wales, which has consumed her and many others’ working lives for 18 months, will collapse next Friday. It remains trapped in a deadlock in the House of Lords, with hundreds of proposed amendments unresolved on an issue that has unleashed intense feelings on all sides.  But her opponents will be allowed little relief.