ir: segment init from inside a proof does not register the thm at qed

Initing a REPL using segment init _inside a proof_ causes problems
after finishing the proof. The '`finish`' closure seems to be gone,
meaning the finished proof is not recorded - i.e. not available under the name the proof
was started with. Furthermore, when it concerns a proof in a locale, `qed` seems to throw
you out of that locale, causing different problems down the line.

Small reproducer available here: 2f2f6fb

Start with reproducer commit 2f2f6fb and reproduce segment init from inside a proof. Trace how the finish closure is handled through qed, especially for proofs in a locale, and confirm that the completed proof remains registered under its starting name without leaving the locale.

Written by the indexing model from the issue text.