Smaller robustness and message fixes

A batch of small fixes: SDK frames with a missing or non-string uuid now get one minted with an error log instead of propagating undefined; a purge helper logs the storage error before falling back to the legacy path; the feedback "payload too large" error was reworded to instruct turning the transcript toggle off and sending again, and now sets a payloadTooLarge flag; the plugin-cache post-install warning dropped its "(or no longer)" hedge; the temp-file exclusion regex now also matches .tmp~xxxxxxxx suffixes; a session-directory probe distinguishes a new "junk" outcome for regular files found where a directory was expected; JSON-schema enum extraction now defends against non-array oneOf/enum values; and the local IPC connect path computes the payload size up front and refuses to connect if it exceeds the size cap.