Undo Trail per phum9679

Undo Trail is a manual, reversible field record for a repair or experiment. Start with a named trail and a description of the last working condition. Each step records one change, the observed result, and a concrete instruction for reversing it. After confirming that a step worked, mark it as the known-good checkpoint. The Reverse Route then lists only the steps after that checkpoint in reverse order, so the newest change is undone first. Steps can be edited, and the last step can be removed after an in-popup confirmation. A finished trail can be archived locally, viewed, restored as the current trail, or deleted after confirmation. The extension does not run commands, inspect the computer, restore a system, read webpages, or automate a repair. It is a record to support the user's own judgment. Trails and their history are stored only in localStorage in the Firefox profile; there is no account, analytics, sync, or network traffic.