POP and IMAP retrieval mutate collections and duplicate UID searches #1684

Problem

POP retrieval mutates the cached message list while selecting results. IMAP UID lookup performs a redundant search and does not handle a missing fetched UID consistently. Repeated calls can therefore observe changed collection state or unnecessary network work.

Prepared commit d495d698822e560b7c640875e6f6ad118247deeb uses owned selections, removes the redundant search and handles absent UID results.

External in-memory retriever models verify repeated POP calls, IMAP command counts and missing UID behavior. The unchanged 1,837-example suite passes on both Rubies; no network server was used.

CONTRIBUTING requires regression specs, so this report does not open a source-only PR or modify tests.