Honor ctx cancellation in object translation

translate() is the single recursive entry point invoked from the tree-entry, commit-parent, tag-target, and message-ref loops. Check ctx.Err() at the top so Ctrl-C during a kernel-scale conversion returns promptly instead of running the whole DFS to completion. ctx is plumbed through newTranslator and stored on the translator (signatures of the many translateX helpers stay untouched).